Moving as silently as he could, Mark made his way to his sister's bedroom. The reason for the 13 year old mouse's stealthiness was that his sister wasn't alone in there. Their parents had gone to a wedding for the weekend and had decided that Stacey being 18 was old enough to look after her brother. The mousette agreed to the pro bono babysitting job on the condition that she could have her girlfriend over. His parents hadn't been gone for an hour before Becca arrived, the tomboyish hyena stopping only to say 'Hi' and muss his hair before grabbing Stacey's hand, after which the two rushed to his sister's bedroom, emerging several hours later only long enough to order pizza. After they all ate, the two once again vanished, leaving Mark alone in the living room. A few years ago, this would have been the ideal arrangement for the mouse boy, since it meant having the big-screen TV and the gaming console all to himself, but recently he'd started thinking a lot about girls. He knew what they were doing up there. Alright, he didn't know, exactly, but he had a vivid imagination that supplied him with a number of possible scenarios. 
                It didn't help that Mark had a crush on Becca. In fact, the image of the busty hyena bending down to say hello to him earlier that day, her c-cup tits straining the low-cut tank top she wore, kept him from enjoying his games, and had set him on the Forbidden Path. He made his way up the stairs as silently as he could, sneaking down a dark hallway toward his sisters room. He could hear music playing inside, quiet, but loud enough to mask his breathing. Fortunately, it wasn't loud enough to drown out the sounds the two girls were making, and pressing his large ear to the door, Mark could clearly hear the two of them making out, as well as the occasional giggle. 
He wasn't sure how long he'd stood there, ear glued to the door as his brain provided the missing visuals for the sounds he was hearing. At some point they had changed, and now he was mostly just hearing one voice moaning and gasping. Judging from the high pitch, he guessed it was his sister's, a suspicion confirmed when he heard a distinct squeak. Listening was no longer good enough, so Mark very gently tried the doorknob, discovering to his delight the door wasn't locked. His sister had obviously expected her many previous threats to act as a sufficient deterrent, and they probably would have if Mark was still thinking with his brain. Turning on his phone's camera and clicking the record button, the mouse opened the door just a crack and slid the phone through just far enough to get a good shot. He could only see part of the screen, and the visibility wasn't much since the only light in the room was a bedside lamp, but it was still enough to literally take Mark's breath away.
His sister was naked and lying on her back, her legs spread and dangling off the edge of the bed, the view of her crotch blocked by Becca's head as she knelt there, holding Stacey's thighs open while her mouth went to work. The hyena had taken her bra off as well, although standing behind her Mark couldn't get a good look at her boobs, settling instead on the view of her toned ass, partly obscured by a pair of red cotton panties.
Mark couldn't believe his luck, thinking this was too good to be true. And he was proven right when a new message notification rang out from the phone he was holding, startling both him and the two girls. Hands shaking in panic, the mouse's first thoughts were of deleting the damning video, rather than escape. He'd just managed to get rid of it before the door swung open to reveal Becca's figure, silhouetted in the light of the table lamp. Mark looked up at her and his expression of surprise and terror must have amused the hyena, since her lips spread open in a grin which might have been disarming, if it weren't for the teeth.
Before Mark could utter an apology or a denial of guilt, Becca had grabbed him by the ear and dragged him inside.
"Lookit what I found outside your door!" Becca announced, dragging the terrified boy inside. If Mark hadn't been scared witless, he might have noticed his sister had just hurriedly put away a small wooden box, covering her breasts and crotch only after it was done. If he'd been older, based on the smell in the room, he would have known the box contained rolling papers and a baggie that was lighter now than it was before Becca came over.
"I-I was just... I mean I wasn't..."
"You weren't what? Spying on me? Oh my God, when mom and dad get back you are so dead!" Stacey's proclamations of doom were interrupted when Becca walked over to her and started whispering something in her ear. The mousette listened carefully, her expression softening, much to Mark's relief. Then she covered her mouth to suppress a snigger, and Mark went back to being terrified. After some more sniggering he noticed his sister blush, which confused him regarding how much trouble he was in. After the hyena was done, Stacey got out of bed, and started rummaging in her closet.
"Now Mark, I've managed to calm your sister down, but you have to understand that what you did was very bad. You interrupted a private moment between two girls..." Becca explained, sitting down on the edge of the bed to get on eye level with the mouse, because she needed his attention. She also folded her hands in her lap so that her arms were pushing her tits together, because she also needed that tent in the boy's pants to stay up if her plan was to work. Overthinking would ruin her scheme, and she knew just how to keep a male from questioning things too much. 
"... but, it was also wrong of us to leave you all alone. So here's the deal - I'll make sure Stacey doesn't say a thing to your parents, and you can stay here with us tonight..." she tried not to laugh at the rapturous expression Marks face took on at hearing that last part "... but, this still has to be a girls only night."
                Mark was confused, in no small part due to the fact that his blood-deprived brain was using all its' resources on processing the visual signals his eyes were sending, which definitely wasn't helping the blood supply situation. A disturbing thought wormed it's way in and Mark remembered what his sister had threatened to cut off that one time she caught him peeking at her while she was showering. When Stacey happily shouted out "Found it!" from the depths of her closet, he was half expecting her to emerge with a hatchet. When she came out holding one of her old summer dresses, he was relieved. Briefly.
                "You mean you want me to...?" Mark broke of and just stared indignantly at the dress.
                "We promise we won't tell a soul! Just pretend to be a girl one night, and you can have some girl's only fun with us!" Stacey said with a grin, dangling the dress in front of her brother.
                Mark considered his options. He could do the dignified thing and turn them down and go to his room, head high and balls blue. Or, he could pay the embarrassing price for a night of hedonistic ecstasy.
                "P-promise you won't tell?" he asked nervously as the girls made zipping motions over their diabolical smiles.
                The two girls almost pounced on him, quickly getting rid of his t-shirt and sweatpants, his underpants being the last to go. As the floral-print summer dress was pulled over his head, Mark was glad that at least he wouldn't have to wear panties. He was kinda embarrassed at how well the dress fit him, sliding easily over his slim frame, although was a bit short, the hem coming down just enough to cover his butt, providing he stood straight. Stacey must have been twelve the last time she wore this! To his disappointment the girl's weren't stopping yet, Becca brushing his shaggy blond hair to one side while Stacey rummaged around a drawer, gushing with glee as she found what she was looking for. Mark's hair had been pinned in place with a butterfly barrette and the girls stood back to admire their handiwork.
                "Perfect! He doesn't even need make-up. He could totally pass for a girl!" Stacey exclaimed triumphantly.
                "Aww, I think he looks cute." Becca cooed, making Mark blush even more than he already was. "But we can't keep calling him Mark. He needs a girl's name..."
                "Ooh! How about 'Marcy'?" Stacey suggested.
                "OMG! He's so a Marcy!" Becca said, her and Stacey collapsing into a giggling fit, not so much out of cruelty, but because they'd spent most of the day getting high and were pretty stoned by now.
                "OK, Marcy, a big part of being a growing girl is exploring your body." Becca said after she got her breath back, trying to sound serious. "Now, you don't really have a girl's body, but Stacey does, and since she's your sister she's kinda what you would have looked like if you'd been born a girl. So you'll be exploring her body!"
                Mark (he was damned if he'd think of himself as Marcy!) looked over at his sister, who was now sitting on the edge of the bed. Her legs were parted and she was blushing slightly and biting her lower lip, looking quite eager. He considered things for a moment. What Becca was suggesting was more or less incest and although he was secretly attracted to his sister, part of him did feel it was wrong. However, since so many things about tonight were wrong, they might as well be wrong in his favor!
                Stacey leaned back a little, supporting herself with her arms and trying not to snigger at Marcy. At first she figured she'd go along with Becca's plan just to teach her annoying brother a lesson, but looking at him dressed-up like a girl was actually turning her on. The tent in the front of the dress did kinda spoil the illusion, but if she didn't mind her girlfriend having a little extra between her legs, why should she mind if her little 'sister' did? 
                "C'mon Marcy, don't be shy! Start by feeling her breasts." Becca urged, guiding the entranced former boy to his sister's waiting body. She watched intently as Marcy slowly raised her arms, her hands shaking from excitement before coming to rest on Stacey's petite teats. She was surprised by the needy moan coming from the mousette as her new sister gently fondled her breasts.
                "Ooh, hear that Marcy? You're a natural! Now kneed them gently and every now and then give her nipples a light tweak..."
                Never taking his eyes off his sister's grey-furred chest, Mark nevertheless followed Becca's instructions, further encouraged by the sounds coming from Stacey. He felt her nipples stiffen and gave them a gentle squeeze between his thumb and index finger every now and then, the older mouse squeaking every time he did. He felt Becca's hand on the back of his head, pushing his head towards his sister's breasts and taking the hint he gently flicked his tongue over a pert pink nipple. Judging by the sounds the mousette was making he figured he was doing a good job, and after a few more licks he took the nipple into his mouth and sucked it gently.
                Stacey moaned feeling Marcy's lips latch on her sensitive breast. She looked down, meeting the eyes of her little sister as she suckled her teat, replacing Becca's hand on the back of her head as she held her close. She'd never been even remotely attracted to her brother, or other boys for that matter, but she couldn't believe how this 'Marcy' thing was turning her on. She lowered her head to plant a kiss on her little sister's forehead, petting her hair and guiding her to her other breast a few moments later.
                Becca sat behind the pair, admiring the sight of the two mousey siblings getting intimate. After a while both sister's had stopped to catch their breath and the hyena figured it was time to move things along.
                "Good girl, Marcy!" she said, noticing that the mouse hadn't even flinched at the mention of his new name "Now it's time for you to learn how girl's get each other off..."
                Becca moved over to Stacey's side, putting a hand on her knee to open her legs. It seems Marcy had done a better job than she thought, the mousette's legs flying open at her touch to reveal that the pink slit which the hyena had diligently licked clean was once again a soppy mess. Becca couldn't help but grin at Marcy's wide eyed expression at seeing her sister's arousal.
                "Now Marcy, this is a girl's pussy. It gets wet like this when a girl is good and horny and ready to have it played with. These are the lips..." she said, parting the mousette's slick cunny "... and this is the clit. Now go ahead and use your fingers on and in her pussy, but be very gentle with that little button.' Becca finished. She dragged a finger over Stacey's nub, making the mousette whimper needily.
                With exaggerated slowness, Mark reached out and touched his sister's pink cunny, making the mousette twitch at his touch. He was aware of his by now painfully erect shaft and hoped that if he did a good job either of the girls would take care of it for him. He carefully slid one finger past his sister's folds, marveling at how it seemed to clench around his digit. Gently and slowly, he slid his entire finger inside, probing around Stacey's warm tunnel until she let out a sort of high-pitched whine. Encouraged by that sound, Mark continued to rub his finger around that same spot inside her, making the mousette squirm as her breathing became rapid.
                Gaining confidence, Mark inserted another finger between his sister's lips, watching with awe as her pussy stretched around his two digits. He started to pump them in and out of her, slowly at first, but picking up speed as the mousette's vocalizations became more and more needy. Remembering what Becca told him, he gently brushed his thumb over her little nub every now and again, which seemed to be all Stacey could take. Collapsing on the bed, his sister gripped the sheets tight, biting her lower lip as she came. Her pussy clenched rhythmically around Mark's fingers, soaking his hand in her juices.
                "Way to go, Marcy! I don't think I ever got her off that fast just by fingering her. Now on to the next lesson..."
                Hearing those words, Mark instinctively started to climb up over his sister, hornier than ever and teased to the brink of madness by the sight of the naked mousette writhing on the bed in post-orgasmic bliss. A firm grip on his shoulders stopped him, and he felt himself being lowered into a kneeling position between his sister's twitching thighs.
                "Where do you think you're going, Marcy? You have to act like a girl while you're here, remember?" Becca said with barely concealed sadistic glee.
                "But..." Mark asked, looking pleadingly at his ignored erection.
                "Don't worry, I'll take care of you. Now put that mouth to better use..." she finished, placing a hand on the back of Marcy's head and guiding it to his sister's soaked crotch.
                Mark gave in, noticing that Stacey had managed to prop herself up on her elbows and was already looking at him expectantly. Resigning himself to his fate, he crawled towards his sister's honeypot, the smell of her fem-cum making his neglected shaft twitch forlornly. He gave her slit a few experimental licks, finding the taste of her arousal strangely alluring. He used his thumbs to spread the plump pink lips and started dipping his tongue further inside the mousette's pussy, making her squeal with delight.
                Becca took in the sight of the incestuous cunnilingus before grabbing her purse and digging out a small tube she always brought on her dates with Stacey, in case the mousette was in the mood for something 'special'. She slipped of her panties, freeing her mottled black and pink shaft. The hyena squeezed some of the clear liquid on her 'clit' and gave it a few strokes as she contemplated Marcy's upturned bottom. She knelt beside the 'girl', and lifted her dress out of the way before squeezing some more lube on a finger. Before Marcy could tell what was going on, she gripped the base of her tail gently but firmly and touched a finger to the now exposed pucker.
                Mark's head shot up with a shock at the sensation of something cold and slimy touching him back there. Looking back, he was in for another shock - Becca was kneeling behind him, an 8" shaft protruding from her groin as she started prodding at his tail hole, her grin clearly revealing her intentions.
                "Becca! What are you..." was all the mouse managed to say before his sister grabbed his head and guided it back to her needy muff.
                "Last milestone, Marcy! Sooner or later every girl needs to have her pussy stuffed. And since you don't have one of those, we're going with plan B." she said, slipping her finger in as Marcy squeaked into her sister's pussy.
                Mark's cheeks were burning, both pairs actually, as his sister's girlfriend pushed her finger all the way inside him, his tail hole futilely trying to expel the invader. He tried to protest, tried to wriggle out of this, but his sister had a firm grip on his head, and just kept pushing his muzzle deeper into her crotch, obviously anxious for him to continue his licking. Meanwhile Becca had started pumping her digit in and out of his slicked pucker, her other hand stroking her pseudo-cock. Resigning himself to his fate, Mark started to lap at his sister's sex, figuring it would be best to get this over with as fast as possible.
                Becca kept fingerfucking Marcy's 'pussy', the mouse's tail still firmly in her grip. She had to admit she had a very girly looking butt, especially with a flowery sundress bunched up around her waist. As much as she enjoyed the sight, Becca decided it was enough foreplay. Still holding the mouse's tail up, she got behind her and lined up her shaft.
                "Get ready to become a woman, Marcy!" she said with a grin and thrust forward, the blunt end of her pseudo-penis penetrating the former boy's 'pussy' as the poor mouse squealed into his sister's cunt. Becca stopped for a second, giving both Marcy and herself time to adjust. The mouse's pucker was incredibly tight and kept clenching around her sensitive rod, making the hyena shiver from the stimulation. Groaning with pleasure, Becca steadily inched her shaft deeper and deeper into Marcy's pussy until her hips touched the mouse's cheeks. Keeping herself buried deep, Becca reached around and brushed her hand along Marcy's small shaft, the appendage twitching at her touch. Muffled as it was, the sound the mouse made was definitely not one of pain or discomfort.
                "Ooh, Marcy, you like having your clit played with?" Becca teased, giving the underside of Marcy's shaft another gentle stroke, making her whimper into Stacey's pussy.
                After being teased for so long, every touch of the hyena's hand sent a jolt up Mark's spine. After a few more strokes, he felt Becca take his throbbing shaft in a very light grip, and then stop. Desperate for stimulation, the boy started to hump into her hand, becoming embarrassingly aware that every time he did he slid off and on the rod buried in his ass. Still, he couldn't help himself, the hyena's soft palm feeling like heaven, especially after the pain of his penetration started to fade. The boy started becoming aware of a different sensation coming from his behind as the huge clit throbbed inside him. It seemed every time it hit a certain spot, his own cock would twitch, adding to his pleasure. As he continued to eat his sister out, his moans and squeaks joined hers and Mark felt himself blush at how similar they sounded.
                Becca just stood still, enjoying the sight of the no longer virgin mouse fucking himself on her cock as he humped into her hand. The feeling of his boy-pussy sliding over her cock, his girlish butt hitting her hips every time he impaled himself was sweet agony, but she was determined to give her a fucking to remember. Putting one hand on the mouse's hip, she drew herself out, making Marcy moan like a whore as her clit slid almost completely out of her ass. With a grin, the hyena gave a thrust, pulling the mouse back towards her as her hips slapped her butt, her shaft once again impaling the 'girl'. She kept this tempo up, the room filling with Marcy's girlish squeaks as the hyena began to truly fuck her. Every time she thrust, the hyena gave the mouse's tiny cock a little squeeze, the motion of Marcy's body serving to stroke the immature length. She figured the former boy was enjoying himself at least somewhat, since his little 'clit' was now leaking pre all over her hand. Looking at the pucker now stretched lewdly around her own rod and glistening with lube, Becca felt her own orgasm hit her. She barely missed a beat as her pussy clenched, squirting her juices all over Marcy's little sack. Panting, the hyena kept up her pace. Not being an actual male, Becca could keep her 'erection' through several orgasms, and she was determined to make the mouse cum.
                Stacey kept a firm grip on her sister's head, alternating between watching the blushing, squeaking mouse eat her pussy, to the sight of Becca humping into her shapely little butt, her large tits bouncing with every stroke. She saw the hyena clench her teeth and knew she must have cum, knowing from personal experience that it wouldn't slow her down. She knew she was close to her own peak, needing just a bit more to get her off. She put her other hand on Marcy's head and gripping it firmly pushed her sister's muzzle into her quim. That in turn pushed her over the edge and her legs snapped closed, burying the smaller mouse in her crotch as her cunt drenched Marcy's face for the second time that night.
                When Stacey pushed his face deeper into her slit, Mark got ready for another torrent of his sisters juices, but was taken by surprise by her thighs coming together and pinning his head in place as her pussy flooded his mouth. Once the flow stopped, Stacey collapsed back on the bed, her legs snapping open and her hands no longer on Mark's head. While he was glad he could breathe again, there was nothing muffling the girly sounds coming from his mouth as Becca continued to pound his ass. He started to hump into the hyena's fist again, desperate to get a little extra stimulation.
                Becca could feel the mouse was close, and seeing Stacey had propped herself up again she decided to give her girlfriend a show. Bending over Marcy, she hooked one arm around her chest and pulled the mouse up. Her other hand stayed on his little cock as she kept fucking his ass. Stacey let out a little giggle and Becca imagined what she was seeing - her little brother, with a butterfly barrette in his hair and her honey all over his face, blushing furiously as he was about to cum from having his butt stuffed. She felt Marcy's 'pussy' clench around her and gave his shaft one last rub as the mouse came, spurting his seed all over his sister. When his cock stopped twitching, the former boy melted back into Becca's embrace, his pucker still clenching around her as he tried to catch his breath
                Becca gave the mouse's ear a playful nibble before sliding out of him and helping him to the bed. Stacey was wiping her brother's cum off her fur with a finger, and as she held the spoo covered digit to the mouse's mouth both girls giggled as Marcy obediently licked it up. After a few more mouthfuls, they cuddled with her a bit before the mouse groggily excused herself and got up on wobbly legs. As the dazed boy made his bowlegged way out the door, Stacey called out after him.
                "Remember Mark, my room is still off-limits to you, but Marcy can come and join us anytime she wants!"