Offline/Openly jerk off next to your sister
Now that you know she knows, you jerk faster. Who were you kidding? You couldn't be nearly stealthy enough to do this without her catching you. But knowing you no longer need to be really sets you off. She knows exactly what you're doing, and maybe has for a while. But she hasn't gone anywhere. She isn't complaining. So you go full tilt.
Abandoning any subtlety, you stand up your cock and jerk off at full speed. The folded edge of your sleeping bag that barely covered you before now slides away as it bounces against the back of your hand. While tightly gripping one of your sister's pert little ass cheeks, you feel precum wet the fingers of your fapping fist, your glutes already clenched with anticipation.
Your body shudders a little as you pummel your meat, letting out a few short grunts through your nostrils as you quickly power to a productive finish. You don't last long, climaxing after a handful of heavy exhales, and apply a vice grip to your cock trying to hold back the flood, but you're too worked up. Even though you're squeezing really hard and pull your cock toward your body, you can't keep your hand still either. You're compelled to keep pumping through your orgasm and so you can't stop the gates from bursting. After a few seconds of struggle you blast your load in a vertical strip up the center of your stomach, laying ropes of cum between your belly button and your sternum.
God does it feel good to get that out. So much tension and frustration floats away like magic and your exhaustion sets in immediately, your arms falling out to either side even as your still twitchy cock pulses the rest of your orgasm onto your pelvis and dribbles down your shaft.
You sure didn't think this through. You're not sure what to do about your cum. Let alone what to say to Emma. But after you close your eyes and let out a few deep breaths, trying to give yourself a minute to recover, none of it matters anymore as you quickly fall asleep.
The cabin is already well lit by the sun when you open your eyes. You slept like a rock and are once again as hard as one, feeling pretty good as you wake up. But as you try to stretch you notice a few things.
You first notice that you can't move freely as your sister is using your arm as a pillow after rolling onto her back. Then you remember that she's laying beside you totally naked and you take a moment to gaze down her skinny torso and flat stomach to her prominent bald pussy bump between her legs, her mons sticking out ever so slightly away from her pelvis. Then you notice your morning wood pointing up at you, hovering above your pelvis. That is followed by the cum smeared across your stomach, no longer in a tight tidy strip.
It takes you that long before remembering how you nutted all over yourself last night. You'd be ashamed of doing that in private, but no, you actually let your sister watch you do that. How fucked up is that? Then again... did you let her, or make her? You had your arm around her while you did it, so you were kinda holding her in place. Yeah, that's fucked up all right.
You inspect yourself but it doesn't seem like you got any cum on your limbs. And it didn't drip down your sides, it's painted across your abs. It couldn't have been you that spread it around. That means Emma must have hugged you while she slept and dragged her arms through your semen. That's extra fucked up.
Laying there, staring at your sister's tight little body, and thinking about jerking off last night, gets your A.M. erection twitching. After a full night's rest he sure is raring to go.
But... really? Do you want to do that again? Sure, you jerk off some mornings, so you don't blame your dick for wanting it, but you shouldn't do that in front of your sister... er, okay, you shouldn't do it again... right? She couldn't have seen very much last night in the dark, so openly doing it in the morning sun's clarity has to be way worse. But then again, if she's still asleep... maybe that's not so bad.