I was about 14 a freshman in highschool and my stepsister was 17 or 18, a senior. Like most 14 year old guys, I was horny pretty much all the time.
She had always been good looking, but I just felt wrong looking at her in any other way than a step sister. But, the older we got, the hotter she got and the hornier I got.
One day she picked me up from school and we went home. We were watching Gilligan's Island or something on tv, I was in the recliner. She had changed like she always did when laying around, into a tshirt and boxers. I guess she was uncomfortable, she moved off of the couch and laid on the floor in between me and the tv.
It wasn't cold in there, but I had a blanket over me anyway. The whole time watching tv I was rubbing my cock through my jeans, casually without moving the blanket so she couldn't see.
But, a commercial came on and she was laying face down, looking at the tv, she couldn't see me. I stared at her perfect ass and legs, right there, and her boxers outlined her ass perfectly! So I kept rubbing, staring at her ass. This went on for about another show, and I would play it cool if for some reason she would look back.
Finally, she went to sleep, or so I hope. As quietly as I could, I undid my jeans and pulled my cock out and started jerking hard, staring at her ass, fantasizing about what I would love to do. It felt so incredible, it felt dangerous because of the threat of getting caught. But, in the moment I didn't care. Then, I came and came hard, but I had enough sense to pull my underwear up before I did to catch what I thought was a lot of cum at the time.
Then, I just continued watching tv. She didn't know, and to this day I still jerk off thinking about that.