Mell shifted and huffed as she ground against her pillow, fingers desperately played with her need.
Jay, sat on his bed, late at night and couldn't sleep. He moved his paw to the night stand where he had his phone on charge. eleven thirty... late.. too late to really do anything, but he figured he could take a quick shower to try to clear his thoughts and maybe relax his body. He locked his phone and then stood, retrieving a clean set of boxers, and a towel from his dresser. 
He padded softly into the hallway between the bedrooms closed his door behind him with a click. Moving towards the bathroom he tried to avoid the squeaky board but he lost his balance and ended up landing on the board, "Shit," he silently exclaimed as he caught his balance on the wall between the hall and his sister's room. Mell seemed to be asleep so he continued to move to the bathroom, pulling the door behind him trying to get the latch to catch silently. 
Settling his stuff onto the counter he stepped out of his clothes and turned on the water, settling it for a moderately warm temp where the cool-air had a low hanging steam rolling onto the floor.
She huffs and grinds, reading a forum she'd found a while ago. It was true, or purportedly true, stories from other girls about their sexual experiences, fantasies and questions. 
She was on a thread from another cub about her fantasies about other members of her family. 
Shifting she froze up as she heard the floorboard outside her room creak and a soft curse. Then the shower. 
Shifting the cub though briefly, she'd heard their parents turn in for the night a while ago and could hear their mother snoring faintly. 
Huffing she shifted and dashed silently out of her room, phone in hand she slipped into her brothers room, set the camera to record and hid it on his shelf, then slipped into his bed, sure the darkness paired with the bathroom light throwing off his night vision would give her enough cover.
Stepping out of the shower, he dried off his fur letting his paws run over the soft fur of his chest as he looked in the mirror. His tone chest moving down towards his white belly, not fat but not defined. He was strong, but he wasn't what you'd see in fashion magazines. He smirked as he looked lower at his waist band of the boxers, gripping onto his hips, his iliac furrows present but merely just showing off his bulge. The fabric of his boxers covering his sheath and his healthy balls, tossing his old ones into the laundry he hung his towel up to dry. Stepping gingerly into the hallway he felt his nipples get hard from the temperature difference. 
He looked up at his door, it seemed to be slightly open but he thought that maybe it was his eyes playing tricks on him. Stepping further into his room he took a breath in and he smelled something, sweet and husky, this smell stirred his loins, his sheath throbbed as it awoken underneath his boxers. He stretched letting his arms cross above his head, leaning backwards as he closed the door. He unwittingly giving his sister hidden in his bed an ample show of his bulge and how her scent began to affect his loins, showing a small pre-cum stain even through new boxers.
She pushes into the wall as she hears the shower shut off. 
Moving slowly she moves her hand to smear some of her juices on his pillow, her fingers moving from where they had been between her legs. 
Eyes fixed on the door she watches as Caleb steps into his room, his sheath bulging in his boxers. 
She silently ogles him, she's seen it all before but years ago when he was younger and... Smaller... She suppresses a gasp at the size of him... She's seen him shirtless and peeved on him but this, even in the dark, is the most she's seen of him. 
She tenses up, ready to either be told off and thrown out... Or to pin him and find out what's in those bulging boxers...
He shrugs off the smell in his room and yawns, now relaxed and feeling the exhaustion calling him to sleep. Moves towards his night stand and grabs his sleep mask, feeling that it might help him rest. He straps it onto his head and sits on his bed, smell getting stronger as he settles into his bed. Pulling his covers over his legs and laying back onto his pillow, he sighed and took a deep breath in, his sheath responding and growing straining his waistband above his pubic fur. His red length glistening with arousal his own pheromones permeating from their source.
She watches as he covers his eyes and lies next to her, without noticing
She silently inhales his masculine scent as he lies back, the bulge growing and cock starting to slip out from his boxers. 
She admires his slightly soft tummy and impressive bulge, at least to her. Most she's seen being of guys about her age who are hardly as developed as he is. 
Slowly she dips a finger into her nethers and getting it well slicked with her scent, carefully bringing her fingers up to a few cm from his nose she watches as he throbs and grows waiting for his breathing to settle as he passed to sleep. 
Her other hand slowly snakes up to find the switch of the dim reading lamp affixed to the bed head so she can truly admire her brother as her plans come together and her phone can capture it in the best quality.
His ears twitch as his reading lamp switches on with an audible click, his breathing remaining steady as he drifts off to sleep. His cock growing under the boxer's elastic... tenting hard off of his groin, throbbing as if begging her to release it from its cage.
His arm underneath the pillow the other by his side as he shifts in his sleep, turning towards her giving her ample access to his front while he nuzzles into his pillow, eyes masked from the illumination from the lamp. 
He breathes in smoothly letting his chest rise slowly.
Content he's asleep and not reacting to the dim light she stares at him, heart pounding.
She'd not really considered what do do, expecting to get caught by now but hoping to maybe get a show for her phone before she was kicked out. 
But this...
She reached down and slowly hooked a finger then another into the elastic of his waistband, gently coaxing the fabric down, revealing his mostly hard cock, glistening from his pre as it was reviled to her.
Deciding to try her luck she gently pushes on his hip. Hoping to roll him back on to his back so her camera can see and to get better access as her mind races. 
So many ideas and fantasies rushing her. 
She shifts and moves down a bit, already level with his chest by dint of height difference she shifts to get a good look at him as she gently rolls him to his back again. 
One hand gently moves to grasp his seemingly enormous shaft. 
As she does so she reaches a decision, she wants that meat in her. Even if she's not sure it'll fit she's going to try.
Caleb groans audibly in his sleep as he rolls over his cock now standing fully erect, throbbing out more sticky pre. 
His arm underneath the pillow moves to his side as his ears twitch his head moving causing the sleep mask to softly fall from position.
Her breath catches as she sees his full flesh at last. Slowly sliding a hand over it as she explores it and his full balls. Shifting she gently licks it, the taste odd and slightly salty, then opening wider she takes him in, nearly making it half way before she has to stop, she bobs on him a few times, knowing the longer she spends like this the more likely she is to miss the chance for what she really wants. 
Figuring he's slick enough now she moves quickly but carefully, pulling off her sodden panties and flipping a leg over him she starts to grind her virginal pussy against his cock. She grinds and smears her need on him, and his pre on the soft skin of her nethers, only a few soft strands of downy pubic hair covering her "puppy patch". 
Resting her hand with her panties in next to his head she gently starts to guide his tip towards her cunt, hoping if he wakes she can be faster than him
She shifts and positions his tip against her folds, gently rolling her hips against it, she doesn't know if she'll be able to take him, but she's going to damned well try.
She grunts slightly and shifts back a bit as she feels him start to slide in. Her movement causes the mattress to move slightly and shift his head, dragging the mask slightly further off. 
He murmurs in his sleep at the slight intrusion of light into an eye and the movement but remains at rest. 
She gasps and sits up, having to move her hand away from his head to better position herself she gets maybe half an inch before she needs to slow down and breath, his older cock stretching her unprepared body beyond what it's ever been asked to take. 
He's definitely bigger than her hair brush. Smaller than the vibrator she found in their mums' draw but she'd not yet tried it, feeling intimidated by its size, though it was probably only about an average adult and a late teen like him...
She groaned slightly and put her weight down, a full inch and a half of him now in her, she started to gyrate and huff over him, hand at her cunt stimulating herself as she humped him, starting to loose herself to the sensation 
She movements causes his mask to slip further up, both eyes now uncovered, he grumbled and, at a particularly forceful downward hump wakes up with a start, he barely registers what's going on as he starts to form an exclamation of surprise. 
Having gained the next half inch she pauses just as her brother awakes. 
Time seems to slow down as his unfocused eyes and sleep fogged brain register what's happening, some primal part acting first as fight or flight fires, 
Being fully awake and aware she beats his exclamation, cutting off the half formed first mumble before he can get any volume up, 
Her hand shoots forward and clamps over the end of his muzzle, by coincidence it's the one with her panties, the strongly scented crotch of which covers over his nose and mouth. 
Some part of him registers her grip is surprisingly forceful for a cub some years younger than him and quite a bit smaller. 
As she stifles him he's trying to get up, which just results in forcing him further into her, the bulbs of his knot slightly kissing her lips. 
She bites down hard on her lip to stifle a moan.
His eyes widen as his brain suddenly catches up with what's happening. He shifts to try and throw her off but freezes as the signals from his cock, and a moan from his sister, immediately completes the information. 
His sister.
His sister is on him. 
He's in his sister
He's IN his Sister. 
Then his brain clicks, she's fucking him! She's got him pinned and is fucking him!
Or was at least. 
She had her eyes screwed shut and it's making odd twitches as if massively over stimulated and her body is not at all able to grasp how to process it. 
Blinking he takes in the scene as best he can from his pinned position. 
She's straining up over him, paw clamped over his muzzle with sodden panties. The other has a flexing grip on his upper tummy fur, just where it meets his chest. 
He can feel her thighs tense and shiver and her whole mass do the same. 
And then there's the tight head around his cock. 
It's like nothing he's experienced. 
She's so much tighter than Jess at that party, or Becky, or even Bryan or Tall-ie at those others. 
Her breathing is heavy and hard. 
Distantly he realises she just did what took all his partners a bit of time to take in fractions of a second it took his hips to buck has he instinctively tried to get up. 
He's first to admit he's not huge, he's probably average, or a bit over, for his age and probably will be when an adult. But she's tiny compared, her breasts are still forming, just a lumpy bud on her thin frame. 
His hand shifts from his side and gently traces up her thigh, feeling it twitch and shift and the new sensations. He soon reaches her massively stretched out sex. 
The lobes of his knot are just a bit away from her lips, he can feel small drips of warm fluid on them and the sliver of shaft out of her. 
Moving up he feels how widely stretched she is, and how she still has a puppy patch, just a handful of downy pubic hairs tickle his fingers as they trace it, finding her clit. 
It's big. Compared to the other girls he's seen its huge. A distant part of him suspects it's part of that heat syndrome thing (what was it? Gertner-Xao-Hernsburg Syndrome? Whatever, that was irrelevant right now) 
He shifts and frees his other hand from beneath his head and gently takes the wrist of her hand at his muzzle. The other sliding back down to her thigh gently caressing in what he hopes is a reassuring manner. 
Her eyes snap open, head twitching and lower lip grinding between teeth she looks pleadingly at him and managed to gasp out the first word exchanged during this entire time
"Please, no, please let me... Sorry" she's practically begging in the breathy tones. 
He tried to make a reassuring shushing as he carefully moves her paw off his muzzle.
She shudders over him, as she sits up over, his hands guiding to sit up over him, his cock settling as she gets comfortable over him. 
"Shh shh shh it's OK. It's, OK"
His mind races as he tries to think of a way out, or to sort this out. But all he can think is the immense pleasure of being in his sister, she's so tight against him, so cute.
But, he's not got a condom or any protection on... What if he knocks up his cub sister...what if... What if...
She wriggles and starts to hump against him, huffing and grunting softly. 
With a shudder he reaches his conclusion, the desperate pleading sounds of her need, the tightness... And those deeply hidden fantasies...
Gripping one of her hips and her back in each hand hand he flips her over and pins her.
"this may hurt, just, breath and relax" he moves, wresting one arm under her. The other next to her to take his weight.  
"You OK?" He gently nuzzles her cheek as he whispers to her. 
All she can do is grin stupidly and nod.
He holds still over her for a silent while, his entire shaft deep in her small cunt. 
Feeling her start to relax around him he pulls out, slowly and carefully, listening to the adorable sounds she makes and the slight shifts, wriggles as inch by inch he leave her. 
Sitting up over her brother she wriggles and struggles to process all the feelings.
Her nether feel so full and stretched out by her brothers cock. 
She can't believe she's not done this before... And definitely will do it again, to him, maybe her father... Fuck if that dildo is representative of an adult dick...
With a sudden ferocity she's flipped on her back and finder herself staring at her brothers chest, his cock shifting wonderfully.
She lets out a long moan, as she feels him twitch and throb within her. Her young cunt stretching wide and deliciously full, the sensation overwhelming and glorious, of not a tiny bit sore. 
She chalked that up to her brother hilting his large cock in one movement. 
Reaching over to his bedside table he dipped his hand into the draw, quickly searching for a bottle of lube and a condom. 
She was so tight around him, and he so worried of hurting her he really needed to find that bottle he'd been given during PSHE at school. 
Finding the foil of a condom wrapper he went to pull out, but instead was held fast by his sister, legs trying to keep his hips in place and arms wrapped tight around his neck
And then she clenched. 
Hard. 
Only his pointed canine tip remained in her but that sensation! He gasped and sagged, condom forgotten his fingers flexing in the drawer, brushing the lube bottle.
"P... Please... Take me, I need it. I need you..." 
She whimpered up into his ear 
Huffing at her please of need and clenching warmth surrounding him, he grabbed the bottle and squirted liberally over his shaft and around her straining sex, not caring if he got it on the sheets, only caring he gave her the best experience he could. 
Now liberally lubricated he fed himself back in, not bothering to be slow as she whimpered and moaned. 
With a couple more thrusts he added some more lube to help his entry and allow him to really get to work on her.
Pulling out until just the tip was in her young cunt he altered his position, reaching back and hitching her legs around his hips. Looking down at where his cock stretched out her young sex, the slight smattering of freckles across her puppy patch. He thrust once, hard, bottoming out in a single motion, the loabs of his unformed knot pressing against her lips. 
He moan was loud and log, panicking slightly he crained down and took a deep kiss. Forcing his tongue into her small muzzle.
“Shh-shh-shhhh… it’s ok, just… try and keep quiet”
Her tongue tentativly flicked and wriggled against his larger one, filling her mouth as he started to thrust. 
The feeling of her tight, young sex was so warm and tight around him, he knew he’d not last long, though from the sounds she was making that wouldn’t matter.
The feeling of her brothers large cock stretching her out was a glorious sensation, especially now he was slicked with lube. Some faint part of her added a mental note to use lube next time she played with herself. Being so filled, stretched and straining, to take her brothers massive cock. 
His thrusts seemed to get more and more etatic, her hand snaked down to start to massage her buldging sex. Fingers shifting and playing with herself, exploring her straining hole, feeling her brothers thick knot bump against her. 
Fingers dancing over her need she started to wriggle and buck under him. Lightining flashing up and down her spine as she came. 
She bucked and squirmed under him. A primal part pulled her brother closer with her legs inviting him to tie her, fill her with his seed. 
Feeling his little sister cum hard around him he groaned into her muzzle and, with one single strong thrust forced his uninflated knot into her. With a couple of tugs his knot started to swell to full size, locking them together as he started pump rope after rope of thin bountiful canine cum into her desperate, young sex. 
Holding eachother tight they lay there entwined in the most intimate lovers embrase possible. They moaned into eachothers mouths, soon breaking the passionate kiss, he sagged and flopped onto the smaller form of his sister, both panting hard in a wonderful post orgasm glow. 
Holding her close, her rolled to his side and held her, gently nuzzling between her ears, her small body twitched and shuddered against and around him as she came down from the glorious high of such a powerful orgasm. 
Both their breathing soon started to level out as they started to doze off to sleep. 
The consiquences could wait until tomorrow.