Offline/Try to satisfy your sister's curiosity
Trying not to gawk (and failing) as your sister continues to stand in her birthday suit, untwisting her bikini bottoms in a nonsensical way, you figure it might be best to fess up to what you saw, at least partially. You're not sure what lie could even explain why all three of you were peeping through the window of that cabin, so a half truth might be best.
"Well Auntie's not going to tell you anything because she wasn't there," you stress, trying to get the blunder twins to follow your lead. "But... yes, okay, we were spying on Jet. Happy?"
"I knew you were spying," Em replies sassily, fiddling with one of the bikini's knots. Dammit sis, just put it on already! You really would like your boner to go away at some point. Getting hard over the cabin orgy was justifiable, but feeling your cock strain against your boxers while eyeing up your younger sister's immature figure and lithe limbs is frustrating and confusing. As far as inappropriateness goes, you've had far worse thoughts about Beth, and she's even younger than Emma, but Beth is not your sister.
"So," Emma continues, "what'd you see?"
How much should you tell her? You're sorta winging this. Not like you've had time to think very far ahead. "Jet was... doing it," you admit, trying to downplay this fact. But Emma doesn't even look up from her fiddling.
"Yeah. Doing what?"
"You know... 'it'. Sex."
Emma, startled wide-eyed once it hits her, lifts her head. "What? You watched... you saw him doing it? For real? With who?!"
You can't take it anymore. You snatch Emma's bikini bottoms right out of her hands. She frowns at you but lets you take over without complaining. You just can't stand your sister taking so long looking all hot and distracting while utterly naked, so you take out your frustrations on her garment by reaming on the knot she's been fiddling with before realizing that she was trying to loosen it. "Ergh. Stupid thing... Well gee, Em, who do you think we saw in Jet's cabin?"
"Yeah, his girlfriend. Duh," adds Brayden.
"No way! You're serious? You saw them? And they were..." Emma pokes a finger back and forth in the air with a look of dismay.
"Uh-huh! And his penis is huge! Way bigger than Jay's!" Great. Thanks Beth.
"Oh my god! You... shouldn't do that!" admonishes Em while grinning stupidly. The shock wearing off, she makes a lackluster attempt to put her skimpy top on, lazily slipping her arms into it while fully distracted by this unexpected truth you just leveled her with. "You're gonna get in trouble if... But that's totes what you saw? Like, what did you see?"
"Um, I dunno," you mumble, bringing the stubborn knot closer to your face. You're hard enough without picturing that scene again. "Like how you'd think, I guess. They were both... into it."
"Yeah! Jet pushed Hannah over and did this!" Brayden proceeds to lean forward in a half squat and buck his hips back and forth in a comical fashion, one that seems hard to believe and yet way more accurate and graphic than Em could ever know.
"Ew! What? No way!"
"Yeah!" confirms Beth, "And Lizzy did this!"
Out of nowhere the 'prankster' steps in front of you and grabs your full boner through your boxers with both hands. Before you can even comprehend what she's doing, she's already tugging on you with a death grip and a high-pitched giggle, her hands jerking your boxers around as she jerks you.
"Augh! Beth!" you cry, throwing your hands over top of hers. You try to pry her fingers away while still holding the bikini, but she's got her hands locked together and seems determined to keep them where they are so she can show off and share exactly what happened. She sure gets the reaction she was looking for too as Emma is made giddy by Beth's complete violation of your personal space, your sister once again failing at putting something on as she incredulously stumbles in place and her top slips back down to her wrists.
"Wait, what?! Did you say Lizzy? What was Lizzy doing there?!"
"She was pulling on his penis!" Beth gleefully shouts, taunting you while jerking her hands about even faster as proof. You shush her to keep her voice down, holding your hands over hers to impede her though unable to keep her still. But if you're being honest you've already given up on getting her to stop. Why would you? A girl is legit jerking you off in the forest. You're not gonna tell your friends that girl happened to only be 8-years-old, let alone your cousin, but this is the exact kind of story that you'd be stupid not to share with them. And yeah, it's wrong and twisted to let her keep going, especially in front of your respective siblings... but it's also kind of amazing to have anyone else's hands on your junk.
"For serious? Just like that? She just... grabbed him?"
"Yeah, but that was before he and Hannah started doing it," Brayden helpfully clarifies.
"Beth... would you... cut it out..." you state meekly, at least trying to fake that you don't want this.
"But your girlfriend isn't supposed to stop until you're done!" teases Beth. Her face then lights up and she turns to her brother. "Brayden! Come help! Do the next part so we can show Emma!"
"Huh? What next... Oh! Yeah! Okay!"
Apparently willing to embarrass you further, Brayden steps closer, drops to the ground next to you, then tugs your boxers down. Beth lets go of you long enough for your underwear to slide down your legs and send your incredibly stiff cock bouncing up into the air. When she snatches it again, pumping her clasped hands up and down your bare shaft, she giggles and blushes up a storm, proudly showing off something she's never done before; stroke the full length of your diamond dick.
You stare down at Beth's hands in awe. If only she had been open to this last night. But your sister quickly shifts from amused to dumbfounded, staring slack jawed as everything just became much more real, little Beth excitedly masturbating you as your hands drop to your sides.
"Jet was doing stuff too, right Jay?"
"Oh, uh, yeah..." you nod at Brayden before getting his meaning. If Beth is playing Lizzy then the least you can do is play your part as Jet to the fullest. But you're on autopilot at best as you reach around to squeeze Beth's teeny butt. You're nowhere near as smooth as Jet, just in general, but especially so since you have to bend down slightly to better reach Beth's rear. Once within reach you cup her bum over her panties while staring at her little hands pumping your meat haphazardly but wondrously.
Brayden gives a big open-mouthed smile to Emma in an "isn't this good" kind of way. But Brayden hops up when Emma's expression doesn't change. He inspects his sister's backside then leans in and hoists her panties high. Working a wedgie well up into her butt crack makes Beth shriek and giggle and distracts her from her task, but it also gives you full access to one of Beth's tight bitty bum cheeks.
"Lizzy's butt was out," he explains to Emma. Your sister does smirk at that. Wedgies are always funny.
"I like your bum, Lizzy!"
Beth wiggles her backside for your sister's benefit, laughing at herself. With the girl spastically strangling your cock, and your hand cupping her wriggling ass cheek, you're emboldened to fulfill your role even further. Jet did more than just stand there after all.
Tossing the knotted bikini down, you free up your other hand to tweak Beth's nipples a little. It takes only a few pinches before the teeny nubs do their best to stiffen. But Jet went even further, didn't he? You reach down between Beth's arms and slip your fingers inside of her panties, separating her panties from her pelvis. Beth giggles and prances, even yelps, when your digits glide toward her crotch, but she refuses to let go of you even as your fingertips find her baby smooth slit.
Jet did not go this far with Lizzy. He stayed outside of her undies. But who doesn't embellish a little when sharing a story? And so just like that, "Jet's" fingers are pressed directly against "Lizzy's" soft and squishy little pussy lips. And it's awesome. You greedily split her lips with your digits and explore her girlish folds, the supple flesh giving way to you easily as your fingertips find her raw wetness, a far cry from the humid heat you palmed over her panties last night. You should've tried this sooner.
"Buh-B-B-Brayden?! Y-you be Auntie Grace again, okay?"
"Yeah! Sure!" he responds happily.
Beth tries to prance her legs further away from you in an attempt to evade your fingers while she jerks you off, but she holds out until Brayden drops to his knees in front of you before she lets go of your dick and skitters away, not unlike Lizzy. But you don't have a lot of time to ponder how Beth escaped before Brayden grips your dick and starts tenderly kissing your slippery tip, his sister having worked plenty of precum from you by now.
"What!?!" cries Emma, continuing to gawk, but she bounces all the way back to amused by this latest twist. Beth, picking her panties out of her bum, bumps hips with Emma as the two of them begin giggling up a storm watching a boy kiss another boy's boner. Emma regains enough sense to finally put her bikini top back on, but otherwise loses herself to the taboo and absurdity of what she's witnessing. Em laughs and gasps and whispers constantly as she and Beth start nattering up a storm, quietly commenting and joking, especially when Brayden transitions from kissing your member to outright blowing you.
Suddenly you feel extra guilty. You can't even focus on what the girls are talking about. Not only did you fail to hold back any details of what you saw from your blabby sister, but out of nowhere she's now watching you get your dick sucked by another guy. And the guiltiest thing of all is how fucking great it feels. You wish it wasn't in front of your sister, but the euphoria of a small warm mouth slipping down your length does more than enough to counter that. You also wish Beth had the same earnest willingness to suck cock like her brother does, but even if she did you doubt she'd have his natural skill.
Brayden is really into this. He might be trying to "ham it up" like his sister by bobbing his head so forcefully, but it sure doesn't feel like it. His right hand grips you firmly at the base of your cock while his left rests on your thigh, even rubbing your leg at times. His lips gently caress your shaft on all sides, quickly sliding up and down just past your ridge while he slides the underside of your glans over his active tongue. You plant a hand on his head without even realizing that Jet did the same thing with Auntie Grace. Maybe you're imagining it but, unlike the first time he tried this, Brayden has a bit of suction going on today. Did Jet talk about that when Auntie was blowing him? You're not sure where he would've picked that up, but it's quite intentional.
You moan more than once, unable to hide your pleasure. Your sister sure can't hide her own satisfaction, openly laughing at every grunt and sigh that comes from you and Brayden. But you really don't care. Okay, you do a little, but you've never had a proper blowjob before. The only thing that has kept you from cumming already is how nervous you are doing all of this in front of your sister, not to mention in the open not particularly far from camp. But that's not going to stop you for long. You're gonna cum, dammit. Emma already ruined the orgy for you, so you're sure as heck not going to let her ruin th-
"Oh! Emma! You could be Hannah!"
"Huh?"
Huh is right. What the hell is Beth getti-
"Justice?"
Your eyes shoot open. You didn't even realize you'd closed them. When your eyes focus, peering into the trees, you spot your cousin Nicole in jean shorts and a tank top. She's walking toward you holding the hand of Yulia, the bright blondie sporting a simple square neck summer dress.
You release a little yelp of fear and push Brayden off your dick. You and your sister both have your respective junk out and neither of you want to get caught like this, so you both scramble to fix that. You bend down to lift your underwear but practically tear them in half before realizing you shuffled your heels onto the waistband, pinning them to the dirt. Emma rushes toward you at the same time to retrieve her bikini bottoms from your side but trips over Brayden just as he's clamoring up.
A true comedy of errors, Emma careens forward and lands at your feet, knocking your boxers out of your hands just as you were pulling them up your shins. This throws you off balance as the rest of your torso shoots up and you throw your hands onto your sibling's head to steady yourself when you bend forward again. Suddenly Nicole and Yulia are just a few yards away and now your sister is the one crouched in front of your rock hard member, your hands on her head, and she's nearly as naked as you are, her bottoms still at your feet.
"What... are you all doing out here?"
"Nothing! Nothing!" shouts Emma, grabbing the other half of her bikini and ducking away from your hands. " I swear! Okay? Honest!" Rolling onto her back with her feet popping into the air, her legs writhe about as she struggles to slip the bikini bottoms past her toes, putting herself into an even less flattering pose. You know exactly how she feels as you make a second attempt to yank your boxers back up, and yet you kind of enjoy watching your sister freak out all panicked and humiliated. She's the one who caused all this. If she hadn't interrupted your peeping, none of this would have happened, so she deserves every bit of it.