Offline/Jerk off to release some frustration
Wanting to sort yourself out from earlier in the day you head outside, walk out into the thin bush surrounding the cabin, whip out your dick, and start stroking in the breeze. You're swimming in hormones and lacking your normal outlets. Between the sight of Emma's smooth slit glowing in the morning sun and her light dabbing of your bare cock you're finding it hard to not think about your sister now that your dick is in your hands. But a flashlight in the distance puts a stop to your fun almost as soon as you start. You put your meat away and walk over to the trail.
"Justice? That you?"
"Daaad!" you groan. "Stop coming out here! We're fine!"
"What are you doing in the bush?"
"I'm taking a piss! Seriously! Can't I pee?!" you exclaim, throwing your hands up like he never lets you pee.
"Alright, I'm sorry. Your mom was just worried about Emma."
"She's fine. She's already asleep. I can look after my sister you know."
"I know you can. I'll talk to your mother. We'll let you be, I promise. Just don't forget you can grab us if you need anything."
You mumble a halfhearted, "Yeah, I know."
Dad heads back, but that spooked you enough to give up on jerking off for tonight. You head inside, strip down, and take Emma's bed instead. It's the same make as yours, just pink, so it's plenty comfortable. Except for a few hours later when you wake up to find Emma sneaking her way inside, then it's notably more cramped.
You let her crawl in but you try not to tip off that you're awake. She'd probably start talking if you did. She leans her slight body into you so she can zip the bag back up then turns around and nestles into your side. Her arm slips across your torso, hugging your body, while her legs do what little they can to intertwine with yours without disturbing you. Her shirt is bunched up higher than normal from her twisting making it really obvious where her panties start and her stomach begins when she presses herself into your side. That much skin to skin contact against your body is all it takes to bring back your stiffy; you are really horny. You pretend your girlfriend just crawled in with you so it doesn't have to be your sister that got you hard. Once again it takes a while for you to fall asleep.
---
You wake up to your sister's warm body pulling away from you. You don't actually open your eyes, perfectly content to let her go do whatever she wants if it means you can sleep longer. She leaves the cabin for a minute, comes back, then spends some time rifling through her stuff. But after a few minutes you hear a familiar squishing sound very close to you. Squinting, you spot Emma sitting beside you just outside the sleeping bag. Her panties are down to her knees and slime is on her crotch.
Your sis is looking down at her privates, her legs parted, pushing her slime against her mound and stretching it across her pelvis. She does this over and over, stretching it, balling it up, pressing and rubbing it. You only get to observe unnoticed for a few moments before she realizes you're awake. She pulls her panties up most of the way looking for some paltry cover.
"I- I just... I wanted to try it," she stammers. "You tried, and... and..."
You just shrug and roll your head away like you don't care. "Whatever. You don't have to explain it to me. No rules, remember?"
You look back and spot a growing smile on your sister's face. She thinks it over then strips out of her shirt, takes her panties all the way off, and kicks her feet out into a wide stance. You now have a much better view and look on with interest as your nude little sis smacks, prods and manipulates the blue slime coating her privates. Your morning wood is getting stiffer.
A bag of pink slime is pulled from her bag and soon coats her chest. A third bag, this one purple, is also spread across her skin, covering her stomach. Weirdly, if someone told you that your little sis covers her entire body in slime for fun you'd totally believe it. But from the naughty look of joy on her face as she giggles at you, you have to assume this is the first time she's ever done something like this. And she's loving it.
Emma pats, rubs and squeezes herself and her slime for a solid 20 minutes, having a one-way conversation with you the entire time. Finally, apparently content, she pulls it all off her body. The slime covering her chest and stomach is easy. But Bluey is stubborn. After some patting she turns away from you and leans as far forward as she can, picking at her crotch in silence for several minutes with an increasing sense of worry.
"Something wrong?" you ask, already predicting what's coming.
"Umm... I'm worried I got some... inside."
"Then get it out."
"I can't see!" she whines. "And it all feels... slimy." She looks at you expectantly, silence taking over.