Offline/Try to ignore Em, even if she's annoying
You do your best to ignore your sister. You really try. You let her ramble on forever but your lack of engagement only frustrates her. She's tenacious though (or really bored) as she pokes you and annoys you on and off forever. You swear 5 minutes can't go by without her being a pest.
When ignoring her doesn't work you try to tell her off. She's messing up your whole evening. You can't follow the dirty jokes and stories from the adults. Worse than that, wherever you go she follows, and her proximity shuts down certain topics entirely. You can't even sneak any beer for fear of her getting you in trouble. It's as if you're treated more like a kid whenever she's around just from her cooties contaminating the area. But all of your complaining only makes Em more combative.
It all comes to a head when you ask Mom to intervene and make Em buzz off. Mom has to side with you once in a while, right? Nope. Of course not. Way worse than that. She sends you both to bed for fighting. Well... fuck.
You go down swinging, kicking up a huge fuss, but that further justifies Mom's decision, your outburst explained with you being tired. As if. You've only been yawning because Em is so boring.
Sentenced to bed, you drag your feet all the way down the trail. Em brought a flashlight with her for once and walks ahead of you trying to be helpful, apologetic even, but still manages to veer off into the bush more than once. Man is she hopeless.
By the time you get to your cabin your anger has subsided and your mom turns out to be right. You do feel tired. Despite that, you refuse to go to bed yet on mere principle. You need at least a small act of defiance to sate your ego.
You and Em both get changed. Em is just following orders on that front but you wouldn't normally change until you're about to sleep. Unfortunately the cabin is still stuffy from the heat of the day making clothing unbearable. It's not easily remedied either. The windows seem stuck so you flap the door for a while to move some air, but when bugs start to get drawn inside by your flashlights you secure your fortress and strip down to your boxers instead.
Fine, mom. I'm ready for bed now. You happy?
Too tired and disgruntled to read, you obstinately sit down with your sketch pad to doodle by flashlight. Em was perfectly ready to go to sleep but, once she realizes you're not, she sits across from you in her nightie with one of her slimes and starts mashing it between her fingers.
You aren't seated for long when you notice that Em's flashlight is resting on the floor next to her. It's angled toward her body so that she can see what she's doing, playing with her gooey pet, rubbing it into her legs and stretching it between her knees, her feet planted flat on the floor. Since Em is facing your side of the cabin it also doesn't take long to notice that her propped up legs are giving you an unrestricted view under her nightie.
It's a damn good thing she's wearing panties. If she wasn't then you probably wouldn't be able to doodle much of anything because you'd be too distracted. Maybe not the worst thing, but you know you don't want to sketch your sister's cooch in your book. Well... you kinda do, but you obviously have more sense than that.
However, the current view of your sister is sketchable. There's not a thing obscene about it either, just an innocent girl unintentionally flashing her underwear. So it's a bit ridiculous that you find her so distracting. It's just panties, right? What's wrong with seeing a girl's panties?
Your subconscious takes over your doodling. You're still complaining to yourself when you slide your pencil over to a blank space to transpose the image of a young tween sitting with her back against a wall and her knees up. A lot of detail goes into what's under her nightie, but it's not your fault her legs are lit from the side and ripe for shading. It's quite incidental that the underside of her panties are framed by her slim parted thighs so well and that even the top hem of her panties are visible under her flipped up nightie.
They say people can tell when they're being stared at. You wouldn't call your constant glances staring, but Em does pick up on it all the same despite being stuck in her own little slime world. Amidst your diligent doodling you catch her looking back at you, something which startles you both. Her eyebrows initially shoot up but she quickly looks down at her slime, pretending your eyes didn't meet. But catching her eye a second time makes her smile.
"Watcha doin?" asks Em, all bubbly. You sigh and shake your head as you turn back to your page.
"Nothing."
"Yeah? Just some more practicing?"
"Yeah."
You try to keep your eyes down, working on the details of your up-skirt sister from memory as best you can. But a minute later you catch a flash of motion across the dark room and look up to find your sister on her knees... peeling off her nightie. You frown as Em tosses it aside and then lays down on her stomach giving you a profile view of her petite figure. That's not the least bit suspicious.
Although you have to mull it over a moment, you flip the page and start capturing your sister in her new prone position, laying in just her panties. "At least she's not naked," you tell yourself... right as you catch her surreptitiously lowering her panties off her hips.
"Em... I'm not sketching you naked again."
"What do you mean? Are you drawing me?"
"...No."
"Then what's wrong?"
"Well... you're getting naked."
"So? You get naked to go to bed. Now it's my turn," she says with a saucy inflection.
Having got caught, she faces away from you and finishes pushing her underwear down her legs, no longer needing to be too weird about it. Except she still makes it weird.
Stretching out buck naked isn't enough for her because almost as soon as she sets her panties aside she grabs them again. Of course she's not looking to put them back on. Instead she bends her knees, leans to one side, hooks her panties over the arch of one foot, and then crosses her ankles in the air to let her panties dangle from her naked body by one suspended foot.
That's not a very natural way to play with your slime, sis. Nobody lounges about like that. Especially not in front of their brother.
You scowl at Em and she gives a smarmy smile in exchange. It's clear your admonishment won't change anything.
"...Nevermind."
Man, you sure are a weird pair of siblings. Your sister is pretending to not pose naked for you... and you're pretending to not draw her like that. And since neither of you are willing to acknowledge this reality, you both end up pretending for quite a while, Em propped up on her elbows poking at her pet and you holding your pad over your lap to keep your erection down.
Admittedly it's a fun game you're playing. She looks amazing laid out like that. Your sister's skinny bum is squished up into a tight little bubble butt with her pelvis laid flat but her back curving up sharply. You put all of it to paper, particularly enjoying how her panties look like they could slip off of her sole at any moment. It's tricky to capture the angle without making them look like they're already dropping.
Em tries her best to look unassuming and act normal without moving too much. You can tell she's acting though. For one, she's practically silent. Her normal resting state is "can't shut up" so she's at least a little shy right now. She's also still glancing at you impertinently though not nearly as often.
This naughty feigned mutual deception thing you're doing is enjoyable, but even the best games have to end. You do sketch Emma for a while though, much longer than you'd planned, doing a couple quick versions to get all the angles right and then refining one into something that really captures your sister's simultaneously short and lanky limbs, shading the small of her back, tiny waist, and her hair tucked behind her ear.
But your lack of sleep and growing guilt is catching up with you. If you let your sister tease you for too long you'll never get to sleep.
When you put your pad away Em's head slumps a little and her panties fall as her feet drop to the floor. She's not just disappointed, she has nothing to show for stripping down like that, other than embarrassing herself. She definitely wanted to see what you drew, but she can't even ask about it without betraying the shared illusion of what you "didn't" do.
"G'nite," you say as you flick off your flashlight and crawl into your bunk.
"Okay..."
Em reaches down and flicks off hers as well. You can hear her slip into her sleeping bag at which point you confidently slip out of your boxers. It's nice to sleep naked, if only because you've gotten used to it. But tonight there's more than just that.
It takes some effort to wear down your resolve and work yourself up to it, but you eventually wrap your fingers around your erection. You just can't help it. Staring at your naked sister for so long, the entire length of her skinny side lit up in the darkness, Em pretending she wasn't laying herself out for you... you're way too pent up to not do anything about it.
Dick in hand, you slowly jerk inside your sleeping bag, trying not to rustle against the noisy synthetic lining. That's annoyingly difficult to avoid though. It would be easier to throw open your bag and not have to worry about the noise, but you're not your sister. You're still traumatized from getting pantsed in gym class and, despite her promise to keep her mouth shut, your motormouth sister would certainly be among the lengthy list of people you wouldn't want to show your dick to. And it's not like it's pitch black in the cabin, so even being perfectly silent wouldn't be good enough to-
"Justice...?"
Fucking fuck.
"What?"
"Well... I just... I still don't wanna sleep by myself. It's quiet and scary."
You move your hands to your sides. Your dick is so fucking hard. You haven't nutted in days. Is she really going to mess this up too? Are you really going to let her?
"You... Why... I don't... ugghhh... fine."
Although you can't admit it, you caved not despite your boner but because of your boner. It's not that you want your sister anywhere near it, it's just that your dick-brain liked the idea of a girl's body pressing up against you again. It couldn't say no.
Emma crawls over to you and opens your sleeping bag. You try to keep most of yourself covered while she gets herself positioned along your side and then toss the bag on top of her to cover you both. But this is something she doesn't agree with.
"You're too hot," she declares, your body radiating your arousal. And to remedy this, she throws aside the top half of your unzipped bag. Your legs are still under but she definitely pushes enough away for her to spot your blatant erection and you struggle to claw enough back to cover up without making a big deal about it.
But now she's here. The small immaculate girl you sketched on your pad has nestled her naked figure against your equally unclothed teenage physique. And your dick couldn't be happier, swollen and throbbing its appreciation.
That's not true; it could be a touch happier. In fact it's begging for a few strokes. That might be all it needs to spew its load and let your thoughts settle. But there's no way... is there? Could you get away with that? Should you really risk messing up your relationship with your sister? Just for a nut?