I originally thought of this as a M/s roleplay but I imagine it may work for any family pairing too.
My phone vibrates under my pillow waking me up. 2:30 and the house and neighborhood are quiet. I think for a moment, feeling nervous and unsure, but soon my own horniness gets the better of me. I move through the house silently and push on your door. I pause at that damn creak every time. I want to fix it during the day, but I don't want to arouse your suspicion since it's been there for so long. And besides you haven't heard me enter yet. I let out an audible "mom" in case you're awake. Ready with my lame excuse of "Did you hear anything?" if you wake up.
When you don't move, or answer, I move over to your bed. Watching you for a moment, studying for any signs that you may be coming to. I'm nervous and my heart is racing, but for some reason that only makes me more aroused. I count to ten before finally giving in and reaching down and moving your covers. Nights like tonight were the best when you were a bit restless in your sleep and your nighty moved. I always am too afraid to move you, or touch you in anyway.
My hand pushes down my boxer briefs and begins to stroke my cock. It's what I had been waiting for all day. I put a little bit of baby oil on it so my hand would move fast and easy over it. The quiet squishes of my stroking fill your room. It doesn't take me long to get to the point of no return, and in that moment I feel strong and confident. I feel sexy, and alive, and then I cum. I stand there for a moment looking down at you. My hand catching all of my cum. I reach down with my clean hand and pull the sheets back up. Enjoying another successful evening as I walk out of your room pushing the door shut till it creaks and going to the bathroom to clean up and then crawl back into bed.
Let's build a world and characters shall we? I've gotten some questions on where does it go from there. First I see this being consensual. I really want her to wrestle with her emotions going from disgust to actually enjoying it. I want to see how far she can tease him without him knowing she knows. Maybe one night she "falls asleep" with her vibrator laying next to her for him to find under the sheets. Or she starts making comments about the sleeping pills she's taking and how deep of a sleep she's getting. I also want to figure out a logical reveal for the two of them.