Offline/Explain to your sister that she didn't see anything
"You didn't see anything, did you?" you try to say in a threatening tone. Either you aren't very intimidating or your sister is exceptionally aloof right now as the threat goes way over your sister's head.
"Yeah, you were playing, but then Nicole kissed you and put her hand in your pa-"
"Emma, you can't tell anyone," Nicole interrupts. "I'd get... we'd get in so much trouble."
Emma's eyes widen as gears turn in her head. "Are you two having sex?"
"No! No way! Not at all!" Nicole insists. "We haven't done anything. We were just playing."
"But, why were you kissing? And why was your hand in his pants?"
"She was just trying to get her panties," you explain, offering no context.
"From your swim trunks?"
You pull up the front of Nicole's dress high enough to flash Nicole's bare crotch as proof. Your cousin yelps, slaps you away, then meticulously flattens and pushes her hem down. You then retrieve the wham panties from your swimming trunks and shake them in the air. Retrieving her panties was probably all you needed to do to offer your sis convincing proof, but frankly you wanted an excuse to prove to yourself that Nicole was still naked under there.
"These sure aren't mine," you explain.
"But why were they in your shorts?" asks Emma, giving you a suspicious look, as if some crazy conspiracy is underway.
Nicole steps forward and tries to snatch her underwear from you but you dodge her reach. Nicole is really freaked by your sister showing up, but despite your sibling's curiosity you're becoming reasonably confident that you can convince Emma not to tell mom given how interested she is. But Nicole doesn't know that.
"He was holding them for me, that's all," offers Nicole, reaching out again.
"Why? Are you not supposed to be wearing them?" inquires Emma.
"She really shouldn't," you remark with a grin. Nicole straightens up and stamps a foot down, containing her frustration for now. "At least I don't think so."
"Well you're playing a weird game," notes Emma. "Is that why you were kissing?"
Nicole jumps on that thought and runs with it. "Yes! I lost, so I had to kiss him. So gross," she adds, making a face.
"You lost?" Emma looks between you two. "It didn't look like you..."
"She lost her panties," you jump in. "And she has to kiss anyone that takes her panties."
Your sister smiles incredulously. "Really? So if I have them then she has to kiss me? For how long? Like, as long as I have them?"
You give a grin to Nicole. "Sure, that makes sense. Here."
Nicole starts to fidget as she watches you hand over her panties to your sister. Emma giggles, holding up the Wham! panties and taunting her cousin with them. Nicole takes a step forward clearly thinking that Emma is an easy target but Emma steps back in response. Nicole second guesses herself when she sees the evil glint in your sister's eye. Your sis then brings her hands behind her back and steps forward, leaning toward Nicole.
"Now you have to kiss me!" she laughs, already happy to be involved in your 'game'.
Nicole sighs, straightens her hair, adjusts her glasses, shoots you a quick glare, and reluctantly plants a peck on your sister's mouth.
"What was that?"
"A kiss," states Nicole. "Now can I have m-"
"That's not how you kissed my brother. Do it like you mean it!" commands your surprisingly sassy sister.
Pursing her lips, Nicole shoots you another look then moves in for another try. She grabs the sides of your sister's face and delicately brings their lips together. It's a soft sensual kiss that builds with a smoldering heat, Nicole becoming more invested and less pent up as the seconds fly by. It looks even better than the one you were getting, more intimate anyhow. A single giggle comes from your sister's mouth between their lip smacking, like she can't believe this is even happening.
Nicole has a plan though. Their bodies drifting closer together, Nicole slips one hand onto Emma's shoulder, then down her arm, and then reaches behind Emma's back. Your sister isn't as distracted by kissing as you it seems and suddenly twirls away with giddy laughter before Nicole can reach her target.
Emma shakes the panties high in the air. "Neener neener!"
"Emma!" groans Nicky, glancing around.
A safe distance away, Emma lifts her tank top, unbuttons her shorts, then shoves Nicole's panties inside of her own underwear, burying it deep between her legs. "There!" She buttons her shorts again. "You can't have it back until you finish kissing me!"
You can hear Nicole whimper a little. She looked like she was enjoying that kiss, and maybe that's what is getting to her now. Her feet cross and thighs clench while she runs one hand up the side of her arm.
"Okay, well... I can't kiss you if you're over there," states Nicole, clearly wanting Emma to get back behind cover.
"Nuh-uh. You have to catch me!" beams Emma before skipping away, watching you both from over her shoulder before she hops out of sight.
Your cousin startles you with a light punch. "She's going to get us in trouble!"
"No she won't," you assure, albeit less than sure. "If she's playing with us then she'd get in trouble too and she's smarter than that. I hope," you add with a mutter under your breath.
"Well why'd you give her my panties?!" she adds with another punch.
You shrug. "I dunno. It was kinda her idea. Besides, I thought you liked it. Being naughty I mean."
"No! Er... yeah. I mean... I dunno." She stammers and crosses her legs again.
"Okay, okay, I know what will help... you should give me your dress too." She smiles and punches you again but you grab her wrist this time. "Come on! It's a great idea! You'll be invisible!" You egg her on a little more, channeling your dad. "It's worth a try. And you won't know if you like it unless you try."
Nicole breaks away in a huff, trying to look angry. "Stop! It's not funny!"
"Ni-CO-ell!" The two of you turn to see your sister round the opposite side of the cabin from the one she left. Emma parts her legs and rubs her crotch with an inviting look. "I have what you want!"
Nicole shouts back, charging straight at your sis. "You're SO dead!"
Emma squeals and runs, cackling a cute evil laugh. You take off right after them trying to stay close to Nicole. Emma weaves her way through the camp, jumping over small obstacles and racing around recklessly. Nicole has a hard time catching up, but not for lack of trying. She clears the obstacles and sharp corners just as quickly but she also has to keep pushing her light dress down. Her pumping legs and the rushing air help lift and billow the light fabric covering her. And with the air dragging her dress back, simultaneously pulling the fabric tight to the front of her body and creating a void inside the dress behind her, it's becoming easier to see through her scant attire.
Thankfully, despite the ruckus you're making, you don't draw a lot of attention. Kids have been running around all week, so the three of you laughing and darting from cabin to cabin isn't especially shocking to anyone. You're pretty sure you could turn some heads if you took Nicole's dress though...
It's a fun thought but that's all it is. That couldn't possibly turn out well for you. Running around like this isn't exactly low-key, but you certainly couldn't get away with Beth-and-Brayden level antics.
Uncle Mike shouts with a laugh for you guys to settle down and Emma takes that as a cue to find some distance from the adults. She makes a break for the path to your cabin, actually finding it this time. Nicole is having some fun by this point and manages to start gaining on Emma while shouting her plans for vengeance, tromping through the tall grass and skirting past bushes. You've been right behind Nicole almost the entire time but despite being eager to join Nicole you don't really want to stop your sister either, so you've made no effort to pass your cousin.
The cabin comes into view and Nicole finally closes the gap and tackles your sister into a patch of wild flowers a short throw from it. Emma laughs and twists around underneath Nicole until they face each other. You lay next to them, carefully observing the back of Nicole's thighs. You snag the elevated hem of her dress with your fingers and flick it higher to try and expose more of her bum. Nicole is too busy wrestling Emma to notice your fiddling.
"Wait!" your sister shouts, grabbing her crotch. "You still have to kiss me first!"
Nicole pauses, pulling back. "Seriously?"
"Duh! That's the whole game, isn't it?"
Nicky rises to her knees and quickly notices how high her dress is when it drapes away from her waist. She gives you a bashful look then pushes it off the edge of her tight immaculate ass to your disappointment. She then turns her bashful expression to your sister.
"Fine. One last kiss..."