Offline/Offer to make it up to them
"You don't need a nightie. But listen, I ca-"
"How would you know?" she replies snootily, releasing her underwear at her waist with a snap.
"What? We have the same sleeping bag, it's totally warm. You're in panties anyway, you're fine."
"But maybe I'm not fine. Maybe I'll get cold."
You groan at that. "You've already slept naked out here! Why would you be cold in panties?"
"Because you made us go out in the rain! So now we'll catch cold!"
"That's not how that works. But really, why would you be colder wearing panties?"
"Maybe Asha wants a nightie," she continues, ignoring your question.
"Who cares about a nightie?!"
"Actually... I kinda want one," replies your cousin, her arms and legs crossed, still naked beside the door.
"Geez, okay, whatever." You rummage through your things and pull out the second last t-shirt you have, a black band T, and hand it to your cousin. "Here." It's a shame to watch Asha cover her light olive skin, her scant hips, her slight curves, but with all the bickering you mostly wanted your sister to shut up already. The shirt is definitely too big for her, but more wide than tall. So despite being covered she still shows off a lot of leg. And knowing there's nothing beneath it is fun to think about.
"You should be nice to us you know," notes Emma, combing her wet hair. Despite all her noise about nighties, she seems unconcerned to continue standing here in nothing but a pair of panties. Not that you're one to talk, your cock still hanging freely, struggling to hold back a semi. "We're going to be famous one day, and if you're nice to us then we can make you famous too."
"Oh my god, I was trying to be nice! I was literally about to offer you a back rub but you just wouldn't shut up long enough..." Your sis just sticks out her tongue and keeps combing. "That's all I'm offering though. So do you want it or not?"
"What?"
"A back rub!"
Emma smiles. "Oh. Sure!"
Man is she frustrating sometimes. "Okay. Then... lay down."
Emma tosses the comb away and sprawls out on her stomach on top of her sleeping bag. You straddle your sister and sit your butt down on her thighs, but mostly supporting your weight on your knees. Meanwhile, no room for her on Emma's bed, Asha takes a seat atop yours wrapping her arms around her shins and pulling her legs in close to her.
As soon as you lean forward your semi unfurls itself and your extended floppy shaft rests across your little sister's pantie-clad butt. Emma doesn't seem to notice, pulling her hair out of the way and crossing her arms on her pillow to get comfy. You plant your hands on slim shoulders and lightly massage your younger sibling. Rubbing down your sister's small body, her fresh, clean, flawless flesh under your fingers, it takes zero time for this intimate physical contact to stiffen your cock. Obviously you didn't offer this minor bit of hospitality with entirely honorable intentions. Watching these girls dance around for two hours has gotten you all kinds of pent up, so this is just a lame excuse to touch them, and maybe something more. Of course, if they're willing, you're going to take advantage of it.
A fleeting rub down Emma's sides ends up tickling her, so you back off and return to her small shoulder muscles. You work your body into it a little, rolling forward with every upward rub of her back, sliding up onto her neck. This whole body motion lets you grind the bottom of your cock against her covered cheeks and press your balls into the gap between her thighs. She still hasn't commented on it, but it's not like you're 'getting away with' this grinding or anything. Emma has to know that it's your dick resting on her butt and you can see from the corner of your eye that Asha is clearly watching everything you're doing to her friend with extreme focus. You can also still see Asha's teenie hairless slit, just barely, her lifted knees showing off everything under her 'nightie'. You just wish the flashlight was on her instead of propped up toward the ceiling.
Your sis makes some light conversation, telling you to go harder or softer, that you should give more apology back rubs, but it's mostly pretty quiet which is a rare experience around your sister. It also feels like she's making fun of you a little, like she's not taking this seriously. It's not awkward though, just a little tense. Frankly all the more reason to be giving out free back rubs.