Offline/Ignore it; It never even happened
You take a second to process everything and figure it'd be best to pretend nothing even happened. Just save her from the embarrassment of trying to address it.
"Hey. It's... really nice outside. Already getting hot," you comment, rummaging through your bag. Your mom always told you to talk about the weather if you don't know what else to say. "And, uh, if the sun's hot the water should feel good. Did you want to try swimming today?"
"Um, yeah, maybe." She looks over her shoulder at you but sounds a little confused. Hopefully you sound normal enough for her to convince herself you didn't see anything, but she doesn't commit to your swimming suggestion.
Snagging a bag of munchies from last night, you get back in your sleeping bag and manage to have a somewhat normal conversation with Nicole. Any anxiety brought on by you barging in soon dissipates despite neither of you mentioning it. Of course it's never far from your mind, wanting to hang on to the memory. Y'know, for later. You eventually change inside your sleeping bags while you talk - fresh undies are always a good thing - and soon wander into camp.
Breakfast is apparently long since done and the adults in charge of the food aren't interested in pulling anything out ahead of lunch. It seems you stayed up and woke up much later than you thought. You're able to grab a few snacks to tide you over though.
People are starting to spread out more, enjoying their own activities. The camp is in decent shape now and other than a scarcity of facilities and well worn paint it's looking more and more like a real destination, sprawling with tourists.
Hunger abated (sort of), you and Nicole wander along the river. You chat with some older cousins sunbathing along the shore with Auntie Grace and then horse around with some younger cousins playing tag. A game of frisbee starts at some point that you get roped into and manage to have a bit of fun until lunch. Nicole splits off to spend some more time with her mom (her dad couldn't afford the time off) so you join up with your parents and sister.
There are various makeshift benches in addition to folding chairs circling the large fire pit, but a lot of the kids opt to sit on the ground. You manage to steer your family into sitting somewhat close to Nicole but frustratingly still outside of conversation range. What you do manage to overhear is her mention to her mom that she wants to get changed into her swimsuit after lunch.
As the people start to finish up you watch Nicole head off by herself back to your shared cabin. The mere thought that in a couple minutes she'll be stripping out of her clothes in your cabin really excites you. And as creepy as it is, you do kinda want to catch your cousin changing again. You know it's perverted, and moreso since she's your cousin, but something compels you. Even just the idea of a chance of a glimpse of her naked is enough to increase your heart rate, so finding the balls to actually do something like that is making your stomach churn.
Waiting until she's out of sight you hold off another minute, partly building up the courage to go through with this, before excusing yourself and heading after her.
The walk to the cabin feels longer than normal, your nerves the only thing keeping your erection down. You try to walk a normal pace to make sure you don't catch up to her. Approaching the door feels surreal. But you grip the handle, quietly turn it, then step inside without warning.
"Hey Ni-woah!" comes your surprised greeting.
You catch Nicole with her pants down - literally - your cousin standing naked in your cabin with only a single foot in her robin egg blue one piece bathing suit and the other in the air, her pointy little boobs just as striking to you as this morning. She shrieks as soon as you step inside and tries to turn away but instead stumbles over her suit and collapses in a naked heap at your feet in the small cabin. Instantly regretting startling her so spectacularly and wanting to make amends (and maybe wanting some excuse to stay) you apologize and reach down to try to catch her a second too late. Still, you grab her arm to pull her up. She strangely lets you help her to her feet but she continues to stumble over herself as she rises, futilely attempting to step into her suit the whole time.
Your face feels as red as hers looks and you have a semi growing in your pants holding up your naked cousin by her arm. You're surprised to see how great she looks naked. She may dress kinda frumpy but her figure is actually impeccable. If you were a girl you can't imagine why you'd hide it under clothes that don't fit very well. Her limited curves are subtle at best and although she's still stuck between kid and teenager you think she looks fantastic with her full bum and compact little boobs.
On her feet, and a second foot in her suit, she yanks and tugs the stretchy material up onto her body. Trying to give her 'privacy', you both turn away at the same time, just as you spot a wispy fluff of pubes above her privates, barely even noticeable.
"I- I wanted to ask if you wanted to go swimming," you eventually get out.
"Well... yeah! You should at least knock or something! You're as bad as my mom."
"I didn't even know you were out here!" you lie. "I was just coming to get changed first... I mean, I thought you might be out here, but..."
"It's okay," she sighs. You take that as your cue to turn around, glancing over your shoulder first. "It happened, so... whatever."
"I like your swimsuit," you blurt out with jittery nerves, staring at the tight article that hugs her slim frame.
"Really?" she asks, looking deep into your eyes. "Do you really like it?"
"Umm... yeah, it's... really... ti-"
Nicole gives you a shove then covers her face. "Shuddup! I'm just joking!"
"Ha! Yeah, I knew that!"
Her playful shove somehow made your dick lengthen in your pants. That makes you feel so much more awkward, as if this wasn't bad enough. It's not quite regret that you feel, just a strong sense of shame. You really are a perv. Ever since you got hair on your balls it seems. The two of you stand opposite each other, faces red and tension high, for an excruciatingly long series of seconds.
"I'll get changed now," you mumble, gesturing to your bag.
"I'll be outside," she instantly responds.
She escapes through the open door. You quickly strip, fight down your stiffy, and stuff your junk into your swim trunks. This idea was so stupid and yet weirdly gratifying all at the same time. As you exit the cabin you worry she may hate you now, but she seems more or less back to normal as you walk away together. You're stuck in your own head though and struggle to keep the conversation going until you make it down to the river.
You try not to look at her during your walk, perhaps just out of guilt, but once you get to the river she is first to step out into the calm shallow water while you observe from the rocks. She looks gorgeous in her swimsuit, especially from behind. The back is cut low and is squared off almost as if the top half was sewn into a separate pair of bottoms. Thin shoulder straps wrap under her arms to each respective side of her chest leaving the entirety of her slender back uncovered. The legs are cut higher than normal and just expose her hips. That hip-baring design does little to help contain her butt. Overall she's showing off a lot of skin for a one piece. Juxtaposed against her nerdy glasses and long ponytail it's as if she has no idea what her body really looks like in this thing. You wonder whether it's supposed to look like this or if there's something wrong with it, but you're happy either way. You even forget she's your cousin for a moment.
"It's not that cold," she assures you, encouraging you to get in.
The view from the front is almost as nice. The high cut of the legs exposes her hip bones along with a hint of the V that frames her pelvis. Her sides are covered, but barely. The material in front just wraps far enough around the edge of her torso to cover her ribs. But if she was a lot bustier you can picture plenty of side boob peeking out from under her arms. Otherwise it's all modesty up top, the neckline square and very conservative. Since you've seen Nicole's boobs twice today it's not hard to picture them beneath the skin tight blue material stretched over her chest. The suit reminds you more of a leotard than swimwear though, a thought that has you imagining her doing cartwheels.
You join Nicole, racing out into deeper water to submerge the erection growing in your shorts again. It takes some time, especially frolicking with her in the river, but you get your hormones in check eventually. You find swimming in a river is kinda neat. If you tread water you're very slowly pulled around the gentle bend, but thankfully the bend is broad enough and water calm enough you don't risk being swept away. You'd have to swim straight for the opposite side to be in any sort of danger.
The two of you lounge in the water for a while, the day being hot enough that you fully appreciate being able to submerge your body. Soon enough other people join you. One of the sunbathers comes in to cool off. A younger cousin jumps in fully clothed which encourages a pair of little girls to strip down to their panties and wade into the water. You all splash and frolic for a while, the younger kids jumping off a rock that is quickly dubbed the "diving board" and occasionally getting tossed into the air. Then one of the girls loses her panties to the river unexpectedly. You feel another weird twitch in your pants seeing her clamor out of the water butt naked, making a big deal about it before running back to camp calling out for her mommy, leaving the rest of her clothes behind on the beach for no discernible reason.
The raucous sounds from the water gradually draw more people down and after a while it starts feeling crowded. Between the clear lack of suits and towels it seems that not everyone believed Uncle Dave about how good the water was for swimming, but people make do. You and Nicole make your exit, opting to walk around the camp to dry off due to that towel shortage. You manage a few laps before mentioning one of the things stuck in your mind.