Offline/Go along with Emma's idea
You do try to talk some sense into her but she's already stripping off her clothes as she runs away. It seems she wants to go streaking as much as swimming. Of course she doesn't get far. You still have the flashlight and you haven't moved away from the cabin. She runs back up to you, a nude pale blur in the darkness, and attacks your pants.
"C'mon! Hurry uuuup!"
You fight your drunk, naked little sister at first which only sends her into a giggle fit. Having a girl desperate to take your pants off is new to you, and you have to admit you enjoy it. You quickly give in and let her strip you in the wilderness, shoes and all. It feels dangerous as hell to run through camp like this, but maybe it will be fun. Assuming you get away with it.
Emma drags you down the path, skipping, jumping and stumbling along. You keep the flashlight pointed real low, trying to minimize your visibility. As you get closer you tell her to be quiet and turn off the flashlight. The fire is still going but there's only 3 people still chatting that you can see.
Guiding Emma off the trail, you skirt around behind some big trees and try to find a path that won't get you both scratched up without pants. You soon make it around the outside of the camp, popping out just past the cabin closest to the water. Thankfully even that cabin doesn't have a good view of the river with various saplings and smaller trees in the way.
Having made it around, you turn the flashlight back on to make it down to the water faster. Emma runs ahead as soon as there's light, giggling like crazy. Watching her little butt book it through the night while lit up by your flashlight helps your dick slowly awaken through your jittery nerves.
Emma hops down the pebbly beach and charges into the water full bore, splashing and cackling with joy.
"Shush! Sis! Quiet!"
She doesn't listen, but the further out she gets the less noise she makes. You chase after her, tossing your flashlight down next to a rock. She's shorter than you, so where she has to tread water you can still stand. You walk past her and turn around (so she doesn't go out any further) and are about to chastise her when you see a light flash behind the trees.
"Look," you whisper. "That's your fault."
"Maybe they wanna come too!" She replies.
You put your hand to her mouth in an attempt to shut her up. The mystery flashlight doesn't get any closer to you though. After a minute it goes away. Your sis throws her arms around you, followed by her legs, ankles locking behind your back and pulling her naked body against yours.
"You saved us! My hero!" She says, kissing your lips.
"Ew!" You splash the side of her face in protest. She just giggles and does it again. So you splash her harder. Now a game, she kisses you a third time. But after a few more times it's no longer a peck, it's a long intentioned kiss. That catches you off guard. Unable to help yourself, you kiss her back.
The whole thing is actually crazy romantic; running down to the river in secret, floating naked in the water in each others arms, kissing under starlight. Being a bit drunk certainly helps you get past the fact that it's with your little sister, else you probably would've dunked her the moment she brought her face to yours. But this is almost magical, the river still retaining some warmth from the heat of the day, your lengthening cock floating weightlessly, the only sounds coming from the sleepy forest around you.
You stand in the water holding each other for ages, slowly and continually making out, one long extended kiss. The river gently flows around you wrapping its way past your tightly held bodies. You are moving even less than the current, focused entirely on your sister's small lips. Emma eventually pulls back, smiles at you, then sleepily whispers into your ear.
"Jay..." She never calls you Jay. "I just peed."
"What!? Ew!"
Your sis cackles and snorts her amusement while you walk away from the warmth spreading in the water. She sloppily kisses you once more then rests her head on your shoulder.
"Jay... I'm tired."
A little disappointed, but sympathetic, you firmly grab onto Emma's butt - which itself earns a giggle - and carry her out of the water. Setting your sis down on the beach, you grab your flashlight and shake your body to try to shrug off the river, the air now feeling cold.
"Your thing is so long," Emma notes with another snort.
"Yeah, it happens sometimes. Now c'mon."
You're a lot less careful heading back, even though the thrill and excitement has worn off a bit. You could try to be extra stealthy and hike down river or something but that'd be tough without shoes and you'd be lucky to find your cabin at all. So you scamper back into camp, running from cabin to cabin instead of weaving inside the tree line, but you still do your best to stay out of sight. You push Emma ahead of you and down the trail, crouching for the first minute just in case, then walk at a normal pace the rest of the way which gives you plenty of time to drip-dry. A few minutes later and Emma walks straight inside your cabin, leaving you to pickup all the clothing she discarded in front of it.
Wiping your feet off as you enter, you throw a bundle of clothes down and flash the light at your sis. She's laying on her side in her open sleeping bag, her legs curled up, practically already asleep.