Offline/Let the girls take the lead
Your sis spins off the bed and crouches on one side with her slime. Asha glances between you and Emma a few times, but does embrace the 'no rules' rule which she seems to have interpreted as 'no boundaries', if not just 'no choice'. All the way into this now, you stand up to help out and Asha joyfully yanks on your slightly damp trunks. She struggles a bit to free your erection but once it pops out - to her shock - the trunks practically fall the rest of the way off.
Emma has you lay down just as she did, then splits Sparkles in half and shares her with Asha. Emma takes the lead, and Asha follows suit, smashing the slime into your erect cock. Wrapping their hands around your member and pulling it like a joystick, they force the goop to bubble and fart and make all kind of noises much to their amusement. Both girls squeeze you again and again, switching high and low on your shaft and mixing their goo thoroughly.
It's Emma who starts to pump you. She watched real close when you did it before so she has some idea of what to do. She sinks her hands through the goo until her fingers find you and jerks you up and down as fast as she can with both hands. She unintentionally muscles Asha out of the way in doing so, but Asha's more or less content to watch. Emma can't keep up her jackhammer pace for long and abruptly slows into a more steady rhythm. It's still sloppy and uncoordinated, but watching your baby sister do this to your dick is more than stimulating enough. She is extremely focused on your cock though. Apparently this is one of few activities that will actually keep her from talking for a while. Until she notices Asha sitting here not participating. Emma decides to assign her a task.
"Make a fist over the end of it," is all she says. Asha brings her hands together, clasping her fingers and pressing her palms around your glans. Weirdly that's what really does it for you; not your audacious sister manhandling your meat, but to have this nervous girl gently clutch your tip in her hands. But with your sister pumping your shaft along with constant pressure on your head, you blow your wad all of ten seconds later. Your first spurt fires straight up in a little semen volcano then splatters across Asha's goo covered hands.
"Yay!" Emma cries. "We did it!" She stops pumping for a second, gawking at the spectacle, but when the white stuff keeps flowing she resumes her pistoning with a giggle. Asha stares in horror as all this stuff pours out over her hands, but seeing Emma only react positively she doesn't actually freak out and holds her fist in place as instructed. Asha starts giggling soon enough, albeit nervously. After another minute your balls are empty and achy, still trying to fire an empty chamber, Emma pumping a wilting flower. You practically have to kick her to get her to stop.
"This is so... messy. And gross. It got everywhere," Asha announces, timidly pulling her hands apart. Strings of both goo and spunk stretch out into ropes between her fingers.
"It's cum," you inform her with a happy sigh.
"It's what?"
"It's okay!" interjects Emma. "I got it!"
Your sis makes swift work of your thick ropey jizz. She dabs and scrapes and rubs pink slime all over Asha's hands and even up her arms, incorporating more and more globs of your semen into her pile of goo. You assume all that jizz has to be changing the composition of the slime by now, but Emma seems to do a good job of keeping it all together, wrapping the slick mess into a tidy little ball, then sticking it back into its bag.
It's only afterward that you notice both Asha's and Emma's hands still look very shiny. You're now certain that all they did was rub your jizz all over themselves, a thin sheen left behind. Emma is hardly bothered by it, picking at her crotch and rubbing one eye just as soon as the slime is bagged. But Asha knows she's dirty. She looks around, no towels or other suitable toiletries nearby. Too polite to wipe on clothing or bedding and too embarrassed to ask about it, she rubs her hands over her stomach and then against her legs, at best spreading and thinning your cum over her body but far from washing it off. It's really gross and yet weirdly hot to think these kids are wearing your spunk like lotion.
You also want to wipe off the residual cum and slime from your dick so you lean over and snag one of your socks off the floor. Asha follows your hands closely as they clean up your long floppy post-orgasm sausage. She still seems a little off about some of the stuff you all just did together, but given her current curiosity she's clearly loosening up. Emma meanwhile dumps the entire contents of her backpack right next to you on top of her bed.
"Okay! Now it's dress up time!" Emma declares, and the pair of cousins happily pick through the limited wardrobe options in front of them.
Silently backing away to your side of the cabin, you fiddle with your floppy cock and lay down to watch the two bubbly girls, both of whom are plenty hyper from so many naughty and stimulating activities. They prance around doing silly things, neither really wanting to get dressed, wearing everything from panties to slime as a hat (Sparkles is notably excluded), then graduate to wearing shirts as pants. Asha starts that fashion trend, poking her legs through the arms of a t-shirt. Emma laughs and grabs a tank top that she tugs up her legs but it hardly works as alternative legwear. The shoulder straps grab the insides of her slender thighs and merely frames her lips leaving her rosy pelvis uncovered. But even pulled up that high the straps prevent the edge of the garment from reaching her chest leaving her nipples on display as well. Even so, they're both forced to hold the hem up just to keep the flipped articles from falling down their bodies. The girls cackle and carry on by themselves, ignoring the inanimate object named Justice nearby.
With shirts hoisted, panties on heads, and socks on hands, the overzealous girls nearly fall over with laughter then slap at each other with their 'paws'. It's all getting a little too ridiculous for your liking though. With an added dash of guilt over what you just did with your little sister, you straighten out your sleeping bag then wander outside for a piss. But you only just start to relieve yourself beside the cabin when the girls burst out and catch you midstream. They hop to a stop and watch, giggling and whispering to each other. Then Emma drops into a squat and pees through the straps of her inverted tank top. Asha lowers herself to do the same but wildly miscalculates the dimensions of her garment and instead of peeing through the neck hole just lets herself go over the inside of Emma's t-shirt. She doesn't even notice until she stands up and feels wet fabric touch her bum, little streams of pee now trickling down her legs.
Asha looks up wide eyed with a silly grin. "I totally just peed my pants."
"No! You peed my shirt!"
The girls pause before bursting out into a cackle in unison. Asha kicks off the soiled shirt leaving her with just her bikini top again (besides her hat and gloves of course). The two disheveled cousins say goodbye - specifically to your wiener - then skip around the outside of the cabin together making up a song as they go.
It's only mid afternoon. Can you even survive an entire night with them?