Twilight Sparkle let out an exasperated sigh as she exited Canterlot High, trying her best to ignore the various couples gleefully rushing past her, eagerly to spend the rest of Valentine's Day with their significant other. The lavender-skinned girl couldn't help but roll her eyes in irritation, unable to ignore that twinge of envy that bubbled to the surface every now and then.
She normally would have spent the day with her boyfriend Timber Spruce… but sadly their relationship had to come to an abrupt halt after Timber had been accepted into Everton High, which sadly required him to move away. Twilight had originally considered a long-distance relationship, but with the difference in their daily schedules, it became pretty clear that they wouldn't have the right amount of time to make it to work.
Granted, they made an effort to break things off mutually… but it still hurt.
Unfortunately said loneliness only intensified when she discovered her ex had already found someone less than a month after he'd moved.
Some girl named Moondancer.
But it didn't bother her, no! Not at all…okay maybe a little bit.
It's not like this would have been the first Valentine's Day they'd be spending apart… or the fact that he'd moved on rather quickly… for a girl who bore a striking resemblance to her… or the fact that she was still single-
Okay yeah, it bothered her, it bothered her a lot!
‘No use getting all riled up about it…’ She mentally mused to herself as she approached the school statue, too lost in her thoughts to notice the surface rippling ever so slightly. ‘I guess Rarity was right, Summer romances never last…’ She thought as she leaned against the base of the horse statue… or rather… what she thought was a solid surface. “Guess I could just-WOAH!?”
Mere seconds after she made contact with it, the surface rippled yet again and the young girl was swallowed up by the portal before she could even let out so much a surprised yelp.
-----
“Uuugh… my aching head…” Twilight groaned as she picked herself up off the ground, as her blurred vision slowly came into focus, she found herself in what looked like… a library?
A rather familiar library at that.
Taking a moment to shake your head and clear her mind, Twilight tried to fully stand up… only to instinctively plant both of her “hands” firmly on the ground.
…and then she realized that her human parts… were pony parts.
“Oh, guess the portal was still open…” She mused to herself as she turned to face the very Mirror Portal that brought her here, complete with the various mechanical parts her equine counterpart had hooked up to it in order to keep it fully functional.
Naturally one would expect her to panic at going from human to equine in the span of a minute, but after the incident during Spring Break, she’d grown somewhat accustomed to the transformation. It was still jarring, but not enough to give her a panic attack like before. 
Twilight's first instinct was to simply head back through the portal and forget this had ever happened. However, it was at that moment that a spark of inspiration hit her. She distinctly recalled Sunset telling her about how Equestria shared certain holidays with Earth, with Valentine's Day being one of them… maybe she could have a heart-to-heart with her “other self”?
‘It's worth a shot I guess, it's not like I had anything important to do today…’ She thought as she adjusted her glasses. Turning her attention towards the door, the human-turned-equine clumsily stumbled to her hooves (it had been a while since she’d been in this dimension) and carefully trotted into the castle hallways…
-----
“It never ceases to amaze me how you manage to turn our Hearts and Hooves get-togethers into your personal science projects.” Princess Cadence chuckled as she trotted past her sister-in-law, carefully hanging several hoof cuffs from the ceiling of the royal bedchambers.
Princess Twilight Sparkle simply rolled her eyes as she carefully poured a pink fluid from a vial into a beaker full of clear water. “Hey now, I didn't hear you complaining when I shared that gender swap spell last year.” The Princess of Friendship replied as she carefully shook the beaker in a circular motion until the color of the liquids shifted to deep purple.
“Well in my defense, Shining Armor looked adorable as a mare, and it allowed for some rather… interesting experiences,” The Princess of Love chuckled as she slid on her leather boots with her magic. “That being said, are you sure this potion will work sweetie?”
“Naturally, I've tested this combination several times. Thanks to the enhancements I’ve made, the aphrodisiac also contains some stamina-boosting properties as well.” Princess Twilight explained as picked up a bottle filled with green liquid and carefully tilted the nozzle towards the beaker. “Now, just gotta add a few drops aaaaaand…”
As five drops of the liquid dripped into the potion, the fluid started bubbling rapidly, prompting the Princess of Love to take a few steps back. “Uh… Twilight?”
“Relax, this is the expected outcome. There's just-”
*FWOOM*
Before Princess Twilight could continue, several fumes erupted from the beaker in the form of a thick pink cloud, which quickly filled up most of the room. “Okay-cough-maybe I added a little too much… but-ooohh…” She shuddered, a sudden jolt of pleasure shooting through her body as her wings spread instinctively. “Ooohh fuck, this stuff works fast!”
“I'll say…” Cadence sighed, her eyes slowly becoming glazed over as she inhaled the fumes, letting out an orgasmic groan as she briefly lost control of her nerves, her wings twitching as they immediately went stiff. “Sweet Celestia... this is one heck of a potion! I… I-I think…” The Princess of Love soon trailed off, her lips forming a coy smirk as she approached her sister-in-law, licking her lips as she eyed the mare's moist pussy. "Sooooo... how about we get started a little early~?” She cooed as she leaned forward and slid her tongue inside the mare's slick wetness, not even waiting for a response.
"Woah-ho-ho! Oooohhh yeah~!" Princess Twilight purred as her horn glowed, lust overriding her mind as her lavender aura levitated Cadence away from her rear and over to the bed. "Don't get ahead of yourself, we agreed that I'd be on top this time." She smirked as she dropped her sister-in-law onto the bed before picking up one of the strap-ons and carefully fastening it onto her hips, using her magic to keep the dildo rigidly fixed in place.
Cadence bit her lower lip as she laid on her back with her wings spread in an inviting display, a chill creeping up her spine as the Princess of Friendship approached her. However, she then noticed a large amount of the fumes wafting out the still-open door. “Uh, Twily? The door-”
“Oh relax, Spike is over at Rarity’s and Starlight is visiting Sunburst back at the Crystal Empire.” The Princess of Friendship cooed as she slowly slid the strap-on up against her sister-in-law's slit, making sure the tip was just touching her swollen and aroused clit. "The only one who'll get hit with the fumes... is Shining, after he finishes showering."
"Oh my~" Cadence moaned, her cheeks flushed as the dildo rubbed against her moist slit. "Shining might be a little cross when he sees we started without him..."
Twilight simply smirked as she used her magic to forcibly spread her sister-in-law’s hind legs. “Well then, we better get busy before he punishes us~”
-----
Unfortunately, Princess hadn't anticipated an unexpected guest wandering through the castle halls, let alone said guest wandering into the fumes wafting through the halls.
“Geez-cough-what the heck was that??” The human-turned-unicorn groaned as she shook her head, a dizzying sensation spreading through her body as she tried to get her bearings. The effect of the chemicals was almost immediate, her motions going from a standard trot to an almost drunken sway in seconds. However as she unconsciously leaned against one of the doors, it creaked open and she was treated to the sound of running water and someone… singing? 
"Uuuugh... hmmm? Who... who's voice...?" She mumbled to herself as she stumbled into the room… only to bump into a figure taller than her. “O-Oh… I'm sorry, I didn't-”
And she looked up and adjusted her glasses however, she was greeted by the sight of an Equestrian stallion, one that looked rather familiar through the steam and fumes.
His white coat…
His blue eyes…. 
And his shaggy mane with at least three shades of blue stripes.
“Shining… Armor…?”
The Equestrian version of her big brother looked down at her in confusion, but his expression quickly became more friendly as he leaned down towards what he assumed was his little sister, the rather thick clouds keeping him from noticing her lack of wings. “Oh hey Twily, I thought you'd be waiting with Cadence back in the bedroom…”
Confused, Twilight took a hesitant step back, not exactly sure what that phrase implied, only for the effects of the fumes to send another jolt of pleasure through her body, her mind swiftly becoming clouded as her heart hammered against her chest. “I-I… I was… looking for… for…” She tried to speak, but as the sensation reached her loins, an orgasmic moan escaping her lips as she leaned in close.
"Whoa Twi, if you wanted to get started early... all you had to do was ask..." Shining replied, his tone becoming more coy as he gently caressed her cheek... right before leaning in and locking lips with his sister, the fumes having already affected his mind to the point where he didn’t even bother questioning why she was wearing glasses.
"Hmmph?!" Twilight's eyes shot open as another jolt of pleasure shot through her body the moment her "brother" kissed her. Despite every fiber of her being telling her that none of this was right, it just... felt soooo good... she just... just wanted to…
And then her mind immediately processed something. Technically this Shining Armor wasn’t her actual brother… so could it really be considered wrong?
Her body seemed to be five steps ahead of her mind as she instinctively let her tongue slip free from her lips, her lustful desires completely eclipsing her rational thoughts. Shining took advantage of the opportunity, slipping his tongue past her own and wrapping his hooves around who he thought was his sister, their tongues exploring each other's mouths as they continued to make out.
After a few more minutes the pair broke apart, panting heavily as they gazed into each other's eyes. “Wow Twi… you're more enthusiastic than usual.” Shining mumbled in between labored breaths, his cheeks turning bright red as he moved in to kiss her once more.
“I-I…” Twilight gulped, suddenly unable to focus, her body shook as the rush of pleasure shot through her entire being, her breathing becoming more and more ragged before their lips locked once more. However, it wasn't long before the heat between her hind legs became too much to ignore, her mind quickly becoming overwhelmed with pleasure as her juices dripped down her thighs. “I-I... I-I need more…” She stammered breathlessly as she pulled back, her body trembling as her eyes wandered down towards her brother's loins.
"Well, if you insist..." Shining chuckled huskily as he sat down, leaning back as he spread his hind legs wide enough for his sister to take in the sight of his fully erect stallionhood. Twilight's jaw went slack as he grabbed hold of his cock and began to stroke it right in front of her, who watched in an almost hypnotized daze.
"S-So... so big..." She muttered through the haze of her lust, drool dripping from her lips as she slowly approached him, leaning in close and gently brushing her lips against the tip.
“Go on sis, Cadence won’t mind…” Shining smirked as he moved his hips a little, pressing the very tip up against her muzzle.
As she took a moment to admire his stallionhood, Twilight felt a shiver run down her spine as the tip of the stallion’s cock brushed against her quivering lips. “So long” She whispered, shivering in anticipation as she took in his scent, was she really going to take all of that inside her? “S-So… big…” She whispered, clumsily caressing his heavy cock with her soft hooves, her breaths growing heavier as she brought her brother's shaft closer and tenderly licked his blunt unflared tip, shivering as she lathered his slit with her saliva, all the while various conflicting thoughts plagued her mind. 
Was this really okay?
It wasn't incest if it was an alternate version of her brother, right? 
Heck, Sunset and the other “her” dated a human from her world, so technically wasn't bestiality… right?
With that final thought, Twilight let her tongue slide over his tip again before the fumes fully engulfed her mind,
With gusto, she dove in and started dragging her tongue up and down the shaft. 
“Mmmm, sweet Celestia I missed this sis…” Shining Armor sighed, moaning softly as he smiled down at her little sister. The way she “innocently” toyed around with his cock was really setting him off. “Gotta say, you aren’t usually in such a teasing mood…”
With her mind too clouded by the fumes, Twilight couldn’t even bring herself to respond, lovingly nuzzling his shaft before gripping it firmly as she opened her mouth as wide as possible. With little to no hesitation the mare went down on his cock, the feeling of her mouth wrapping around his shaft sending him into a new world of pleasure.
Smiling with pride, Shining Armor pressed his hood atop Twilight's head and started guiding his sister down his throbbing shaft. Little by little she took more of his cock into her mouth, to the point where she could almost feel the tip press against the back of her throat.
Quickly, she started bobbing her head up and down his shaft, as if she were attempting to push his tip down her throat she briefly popped it out of her mouth and gave it a sensational lick from his balls all the way up to the tip. What she lacked in experience she eagerly made up for in enthusiasm, and in a few seconds she took it all back into her mouth and began bobbing her head up and down once more, picking up the pace.
‘Oohhhh good God… this tastes… incredible… mmmm~’ She thought as she pulled back and started peppering the shaft with kisses up from the base, stopping just short of the tip. Opting to pay particular attention to the tip, Twilight licked and sucked, alternating between playful licks and gentle sucks as she closed her eyes and concentrated on pleasuring him, increasing the pace of her motions slightly. 
It wasn't long before a dollop of clear and sticky pre-cum started to leak from the tip of Shining’s cock. Flicking her tongue over his tip once more, Twilight quickly slurped it into her mouth while slowly swishing her tongue around the tip, occasionally licking and kissing it in a loving manner, sliding her hoof up and down the shaft while devoting her mouth to pleasuring just the head.
“Mmmmph, stop teasing me sis.” Shining groaned as he stroked her mane, letting another grunt of pleasure escape his lips as she slurped up more of his thick pre-cum. "...I know you can take more than just the tip."
“Hhmmm?” Twilight gave her brother a quizzical look… only for him to force her head down his shaft with little to no warning, the mare immediately felt her jaw being forced open even wider than she thought she was capable. At first, she was tempted to pull away, but the second his cock glided over her tongue her mind immediately went blank, the taste alone pulling her into a trance and she suddenly wrapped her hooves around Shining’s waist, pulling it deeper into her mouth, slowly inching closer and closer to her throat, soft wet gurgles being the only sounds escaping her mouth. While this wasn't exactly her first time sucking someone off, it was definitely the first time she'd ever taken anything this big. 
“Mmmmpph… hmhmhm…” She groaned around his stallionhood, inching forward just enough for her lips to kiss the medial ring. It was at that moment that her movement halted as she realized she was only halfway there, letting his cock sit in her throat for a few seconds as she opened her eyes and gazed up at her brother.
Noticing that her movements had stopped, Shining Armor glanced down at his sister and gave her a loving smile, gently rubbed the back of her head again, sighing as he leaned back again and let out a low moan, only momentarily taking note of the fact that her warm mouth and throat felt a lot tighter than it usually was. “Fuuuck… I’ve been waiting for this… now take it all in.” He cooed as he bucked his hips upwards, forcing Twilight’s lips to slip over his medial ring.
“Hhmm!? Mmmph! Mmm-GLUCK!!?” Twilight's eyes rolled back as felt the hoof of her counterpart’s brother pushing her head further down his shaft, forcing her to swallow up more and more of his shaft, her body spasming as it started inching further down her throat, the very feeling of the tip of his cock scratching the back of her throat driving her wild with lust. A part of her wanted to push away, but her body seemed to be on autopilot as her hooves simply hung limply at her sides. All Twilight could do was take in deep and heavy breaths through her nose while more of her brother's shaft glided over her teeth, tongue, cheeks, and throat. 
“Gluuuk… sluuck, huurk… GLUK!?” Once again, Twilight found out that she wasn't as in control of her body as she thought, with her suddenly slamming her muzzle down his stallionhood until she felt her lips pressing right up against his crotch, her nose pressed up against his fur, and her chin right up against his balls.
Shining grunted as his cock pulsed, his hips instinctively thrusting on their own, the tip of his cock twitching in her throat and his balls suddenly tucked up against his sister's chin, the pleasurable sensations of her throat clenching around his shaft pushing him over the edge before he had a chance to endure it. “Fuuuuccck…” The stallion gasped as he firmly held her head in place, bucking his hips a few more times before finally shooting a torrent of cum down his sister's throat, completely skipping her mouth entirely and pumping his thick seed directly into her stomach.
“HUURRGLUK!? GULCKKK GURKK!?” With a small jolt of pain snapping her out of her stupor, Twilight's body immediately spasmed as she was forced to swallow his load. She immediately started gagging as she frantically pressed her hugs against her brother’s legs and pulled back as quickly as possible, cum started spurting out of the sides of her mouth the moment she reached the tip.
“HUURRGGHH-GAAAHH!!” Twilight started coughing the moment she released the entirety of his shaft and collapsed onto her back, violently coughing up more cum before desperately taking deep breaths, while subconsciously cursing her lack of a gag reflex. 
However, it wasn't long before the lust-fueled fumes rapidly overtook her mind yet again, each breath she took dragging her back into a lustful stupor. “Uuuhhh… thick… and creamy… haha… h-haaahh…” She panted deeply with cum running down the side of her cheeks and her chin.
“Aaaahhhh yeah… you did such a good job, Twily~” Shining sighed as the last few spurts of cum from his cock finally died down. Despite this however, his stallionhood remained perfectly erect thanks to the fumes filling the room. Carefully getting back up, the stallion sauntered towards his panting sister with a rather coy expression. “But I think we both know you can take more than that~”
With her mind clouded once more, Twilight could only shiver in anticipation as she watched the stallion version of her big brother approach her, his well-built body towering over her small form. The unicorn could feel her instinctively shrink away, but she quickly found her eyes drawn to her brother’s throbbing shaft, the very tip of it now resting snugly against her marehood.
“Just relax sis… you know I'll always be gentle.” Shining cooed while lining up his saliva-coated shaft up against her pussy lips and rubbing the tip around the entrance of her sopping wet marehood, the very feeling sending shivers up Twilight's spine.
And then, with one swift buck of his hips, Shining entered his sister.
“AAHHAA!!?” The mare let out a gasp as she felt the head of Shining’s cock push against her entrance, and yet… there was no momentary spark of resistance or hesitation from her, with her body instead thrusting back almost instinctively. A reminder that she had been a virgin and this was technically her first time… and it was to a pony version of her older brother.
It was possible she might have to speak to a therapist about this later… but for now…
“Mmph, oooohhhh~!” Twilight groaned out, her eyes rolling back as her whole body shuddered from the sheer length of the stallionhood stretching out her inner walls.
As the stallion continued to drive his cock deeper, a sudden feeling of confusion wormed its way into Shining’s mind, he was sure that his sister wasn’t normally this tight. In fact, the last time he felt this much resistance was during their first-
An overwhelming wave of pleasure quickly banished those inquiries from his mind as his sister’s vaginal walls clenched around his shaft. ‘Maybe it's a spell of some sort…’ Was the last semblance of attention he gave such thoughts before they were eclipsed by the feeling of Twilight's inner walls slowly stretching to accommodate the girth of his member. “Mmmyeah… you have no idea how much I've missed this…”
“Haah… h-haaahh… f-fuck! Ooohh good lord!” Twilight moaned out, her eyes rolling back as her body slowly adjusted to his length, her inner walls yielding to his immense girth in a matter of seconds as he started thrusting in and out of her marehood. Then, as if her mind had gone on autopilot, she eagerly wrapped her hind legs around his waist as a rush of adrenaline surgedthrough her body. “D-Duh… d-d-deeper… ooohh god go deeper!!” 
“If you insist, sis!”
Assuming his sister had decided to forgo any semblance of role-play, Shining licked his lips as he furiously picked up the pace, mercilessly driving his hips up and down while forcing the mare against the ground with enough force to make her wheeze. Biting his lips the stallion couldn't help but shudder with each thrust into her almost painfully tight pussy, he had to make a mental note to ask her what kind of spell she’d cast to make it that tight. 
“Hope you're ready, ‘cause I'm not stopping ‘till I've filled you up!”
“D-Do it then! Fill me! Fill me u-OOOHH!?” Twilight moaned out, gasping for breath the moment she felt the stallion's medial ring bump against her pussy lips. It was then that realization dawned on her that Shining was only halfway in, a fact that made the mare shiver in anticipation. ‘Christ! This is how horsecock feels!? No wonder Sunset dragged Flash through the portal after every date!’ Twilight thought as she tightened her hind legs around his hip, her tongue lolling out the side of her mouth. 
The stallion tensed up in response as he felt his sister's pussy tightening around his shaft in a vice-like grip, like a snake attempting to crush its prey, a feeling that engulfed the rest of his cock as he began to rock in and out of her. ‘J-Jeez Twily, y-you’re really making me… w-work for it!’ He thought, grunting as he pumped his cock in and out of her gaping pussy, not stopping until his medial ring slipped inside as well. An orgasmic sigh soon escaped his lips as he felt her marehood completely engulfing his cock. ‘S-So… f-fucking… tight!’
“Gguuuhh! Ooohh fuck! K-Keep going… thuh… thrust harder!!” Twilight gasped out as her body was assaulted by jolt after jolt of pleasure, her hips bucking wildly against the stallion’s in an attempt to meet Shining’s powerful thrusts with her own as fluids leaked from her quivering pussy. "Guh… you're-God… y-you're so thick, it hurts… Ahhgg… don't stop... more… ooohh god, more, please… ow! N-Nnnngh… Ooohh… OOOOH... FUCK YES!!” She continued to shout out as she bucked her hips against him while firmly locking her legs around his waist.
As he felt the resistance of his sister’s inner walls slowly giving away, Shining began thrusting harder, gritting his teeth and forcing his girth into her soft marehood, the feeling of her vaginal walls tightening around his shaft earning a sharp grunt from the stallion.
"Guh… feels… so goooood...” The stallion gasped, savoring his sister's quivering for but a moment before speeding up his thrusts and pressing his weight down on top of her, grunting and gasping out as more of his stallionhood was forced deeper inside his sister, all the way down to the hilt.
"Oh… f-f-fuuuuck yes!" Twilight gasped out, her back arching and her body quaking in pleasure as her brother hilted her, her mind nearly lost to the amount of pleasure assaulting her very body. The sensation of being filled again and again by such a handsome male was nothing short of a dream come true for her blissed-out mind, even if he did look just like her BBBFF… and was a horse. "Shit... you’re-Grrrmph, so big… ughgh! Gahh… aaaaahhhhh… m-more… please, more… p-pluh… p-please-Ohhhhhhhh!!” She moaned out in a fit of ecstasy, her hips frantically grinding against her brother’s as she felt his balls slapping against her rear.
The stallion's hips continued to jerk back and forth as he slammed his cock into his sister's tight pussy, groaning loudly as the slick juices coating her entrance allowed him to enter her at a smoother pace. His hips shivered in delight as he thrust in and out, relishing the sound of his sister shuddering beneath him as her expression shifted to orgasmic satisfaction. 
“Ooh… so fucking… wet…” He grunted to himself, gritting his teeth in pleasure as he felt his shaft slide in and out of her, his mind engulfed in an almost dizzying ecstasy as his thrusts picked up speed, growing more and more forceful.
“H-H-Haaahh! Mmmph…” The mare groaned in pleasure as the stallion continued to thrust and drive his rigid member into her, Twilight could already feel her body contorting as his balls slapped against her ass with each powerful thrust. “Ooohhh… ohhh… f-fuuuck! Fuck… aahh! Yeah! Fuck… aaohhhh!!” She growled out as the stallion’s cock penetrated her at a punishing pace, making her pussy clench tighter around his base, squeezing him like a vice as he continued to pump in and out of his sister, occasionally pulling all the way out to the tip and then violently thrusting back in. 
“AAAAAHHHHHHH! AAGH!!! aaahhhh! Fuuuck! AAAhhhh! AHHH! FUUUCK!!!” She gasped as she felt him finally hit that spot deep within her. Her body was quickly overcome by another wave of intense pleasure, the stallion’s cock plunging within her faster than she could react to the sensation. It wasn't long before her entire body was convulsing from Shining's relentless pounding, orgasmic bliss overwhelming her as the stallion continued to thrust in and out of her pussy as she writhed under him, throwing her head back and squealing as her body writhed against his.
As the stallion thrusted as fast as he could he soon felt his sister's quivering body suddenly seizing up and she inhaled deeply. Pulling away for a moment, Shining Armor was greeted by the side of his sister's blissed-out expression, her eyes rolling back and tongue lolling out of her mouth as her hind legs twitched in ecstasy. 
‘Ahhh yes, the telltale sign that she's about to cum.’He thought, a coy smirk gracing his features as he slowed down his thrusts and leaned forward, not stopping until his lips were near her right ear. “Hope you cast that little spell of yours earlier, because I'm not pulling out until I've poured all of my seed inside you.” He chuckled as he ground his hips against her, his cockhead slightly pressing against her cervix as he leaned in closer.
Gritting her teeth and gripping the stallion’s waist tightly, Twilight let out an increasing stream of ecstatic cries as her entire body quaked under the onslaught of pleasure, her entire body shuddering under Shining’s grinding motion and thick cock as it thrust inside her. 
“D-DO IT!! PLEASE! I-I NEED IT!! F-F-FILL ME!! FILL ME UP-OOOOOH FUUUUCK!!!” She gasped out, her walls once more quivering and clenching around her brother’s thick, pulsing stallionhood as he continued thrusting his shaft in and out of her once more, relentlessly pounding her marehood with all the speed he could muster. “AH! AH! AAHH! AAAHHAAHH!” Twilight cried out, her body shaking with ecstasy as her mind went blank once more, her trembling body engulfed in another wave of intense pleasure as her brother’s shaft plunged into her hungry depths “ I-’M… SO… S-SO-OOFF!! F-FUCKING… CLOSE!!” She gasped out as her face contorted into an expression of animalistic pleasure.
“Then cum, sis.” Shining whispered, his hips shuddering as her quivering pussy rhythmically gripped his cock, almost as if she was trying to milk him for all he was worth. Holding her close, Shining then kissed the top of her head gently before pulling her into a more comfortable position, biting his lip as he felt his balls tighten up and his orgasm fast approaching. “J-Just let… nngh, let. it. all. ouuuut!” He grunted out, punctuating each word with a deep thrust, unable to control himself as the sensation of her inner walls contracting around his stallionhood pushed him over the edge.
“I’M-OH! G-GONNA-H-HHAAAHH!!” Twilight couldn’t help but groan out in excitement as her eyes rolled back, her whole body stiffening and convulsing as her first orgasm of the night hit, a flood of juices spraying out of her pussy and around Shining’s stallionhood, her tight vaginal walls clamping down around his cock even as Shining kept thrusting. 
“MMMM… THAT FEELS… S-S-SOOOO GOOOD-MMMPH?!” She moaned out, only to be cut off as her brother leaned in and kissed her again just as he came. Twilight’s entire body shuddered in orgasmic bliss the moment she felt her brother’s semen erupt into her womb, she could feel the stallion wrapping his hooves around her as his body briefly seized up, firing another load and filling her with his own sticky fluids. 
"Ahh… oohh Shining… thuh-thank you… ohh… it’s... it’s... o-ohh!!” The mare sighed as their lips parted, lost in her own bliss as she felt the jolt of pleasure from her own climax overtaking her, her body tensing up with every fiber of her being as she thrust against her brother, her hips grinding against his with every pump. "OOHH! OH-OH-OHHHH!!” She screamed out, her entire body stiffening under her brother’s weight as he continued to pound his large cock into her, his balls now thoroughly covered in her fluids as more and more of his cum filled her up. “Thuh-thaaaank yoooou~” Twilight gasped out, still breathing heavily as gazed into her brother’s eyes.
“Twuh… Twily… I-I… can’t… I need… mmmph rest…” Shining gasped out, his eyes fluttering as he slowly withdrew his slowly softening cock from within Twilight's cum-filled pussy, his body gradually relaxing as he finally collapsed on top of her.
“Ah… h-haaahh…” Twilight gasped out in bliss as she felt her brother's stallionhood finally slip out of her pussy with a small *pop*, her marehood gaping and quivering as it suddenly emptied his seed slowly leaked out of her and pooled up on the floor. “Ooohh Shining…” Was all she could giggle out as she gave him a gentle kiss on the cheek, weakly shaking and squirming beneath her brother.
After a few minutes of rest, the mare was just barely able to properly gather her thoughts. Practically pleasure-drunk from such an experience, the human-turned-pony clumsily pulled herself out from under Shining’s body and staggered to her hooves, more of her brother's seed leaked from her loins as she clumsily approached the door, leaving the equine version of her brother sweating, panting, and barely able to move. The only thing on Twilight's mind at the moment was finding the Mirror Portal and leaving this dimension before her mind succumbed to the fumes once more, it was already difficult for her to think straight without her mind wandering back to her brother's massive, throbbing shaft-
‘N-NO! F-Focus Twi… fuh… f-fucking focus…’ She mentally protested, leaning against the door to push it open and she glanced back at her panting brother. Biting her lower lip, she quickly yet reluctantly tore her gaze away from the stallion and staggered down the hall, with a drunken sway in each step. ‘Guh… g-gotta… f-find… find the portal…’
-----
“Hey Shiny! What's the holdup? Cadence and I had to start without you!” A familiar voice called out as the doors to the shower room swung open, the steam escaping into the hall and clearing up to reveal a rather irritated-looking Princess of Friendship as she stormed to the room, her mane a frazzled mess, her strap-on drenched in fluids, and a couple of lipstick smears all over her muzzle and neck. “Seriously, how long does it take you to clean yourself up, even Spike doesn't take… this… long…?”
The Princess of Friendship could only stare in confusion and she was treated to the sight of her brother laying on his back, panting as his half-flaccid stallionhood switched with every breath he took, coated in both his seed and what the Princess could only assume with somepony else's fluids. Tilting her head, the princess then noticed a trail of said fluids leading away from her brother and out the door. 
‘Did he fuck one of the maids? I could have sworn I sent them home early…’ the Princess of Friendship mentally mused to herself before turning her attention back to her resting brother.
“Oh well, I suppose Cadence can wait a little longer while I clean you off~” Princess Twilight cooed as she shredded towards her big brother and leaned in close, giving the very tip of his shaft a playful kiss before licking off some of the fluids, turning another twitch and a moan from the stallion. “Now then, be a good boy and let your little sis have a turn~”
“Y-Yes Ma'am…” Shining Armor sighed, too blissed-out to protest even if he wanted to…
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In another part of the castle, the formally human Twilight stumbled through the halls, trying her best to recall exactly where the library was. Unfortunately, inhaling the fumes for so long had messed with her memory to the point where she was pretty sure she had gotten lost. “Ugh… maybe I could find one of the guards and-”
Then at that very moment, the doors right next to her were flung open and Twilight was assaulted by a gust of some familiar pink clouds, the aphrodisiacs feeling her lungs once more as most of the hall was engulfed in it. 
“Oh geez-cough! Not again!” She groaned as a familiar feeling of arousal shot through her body once more, her legs quivering as her loins were assaulted by the lustful need for any semblance of stimulation.
“Back so soon Twily?” Another familiar voice cooed as her body was suddenly lifted off the ground by a light blue aura of magic. Twilight was then forcibly turned around and found herself face to face with a pony version of Dean Cadence, her coat matted in sweat, her mane completely frazzled, her wings spread out and in an almost dominating manner, and an almost predatory grin on her lipstick-smeared face. “Well then, I guess it's my turn give me the dominant one~”
Twilight could only gulp nervously as the equine counterpart of her sister-in-law dragged her into the royal bedchambers before she could even protest or formulate a response for that matter.
It was going to be a long night…