I submitted this story on the old site, so now I'm resubmitting it to this account and adjusting it for the new format.
The first time my sister walked in on me masturbating, I freaked out and tried to cover myself with my blanket. I was only 13 years old, and she was 16. She laughed at my reaction. Then she said to me, "Relax, perv. Ya think I don't know you jerk off?" She had only come into my room to grab something, so while she went through my desk looking for it, I said nothing and stayed under the covers. I didn't exactly know how to respond.
She looked back at me and saw me just lying there. "Really dude, it's fine. Ya don't need to stop," she said. "But...you're here," was my only response. She laughed again. She always found my awkwardness funny. "Not like I've never seen a dick before. Besides, you're gonna get blue balls if ya stop halfway like that." She went back to looking for whatever it was she came in for.
As she did, she bent over, and staring at her ass in her tight jeans shorts brought my erection back. I slowly started stroking my cock again. Eventually my sister found what she was looking for. As she left, she looked back at me and smirked. "Play nice, now!" she said, waving once as she exited my room and shut the door.
After the first incident, my sister would walk in on me masturbating every now and then, without even a knock. Mostly she'd greet me with a casual "Sup, perv?" or something and go about whatever business she had in my room without much more than a second glance and a witty remark as she left. The more it happened, the more comfortable I got masturbating in front of her.
At first I'd still instinctively try to cover myself before continuing, but eventually I wouldn't even break pace when she barged in. It actually thrilled me to masturbate in front of my sister, but for about a year I never got the chance to do it for more than a minute or so at a time.
I finally got the chance to masturbate from start to finish in front of my sister when I was 14. We were watching some action movie, sitting together on the couch, with our parents out of the house. The movie got to a particularly hot scene involving one of the sexy female characters, and I was starting to get aroused. I didn't even think to jerk off then and there. So far, my sister had only ever walked in on me while I was doing it in my room. Whipping it out in front of her would have been crazy. My sister noticed me shifting uncomfortably, trying to keep my erection in check.
As the scene went on and I continued to move around too much, my sister said to me, "Dude, I know you got a hard on. Just take care of that thing before you blow a load in your pants!" She looked at me until I reached for my crotch and unzipped my pants. She turned her attention back to the movie as I took my cock out. She was definitely getting aroused by the scene too, but she wasn't showing it much.
So I jerked off, then and there, on the couch next to my older sister. I kept watching the movie as I did, but I kept glancing over at my sister. The scene hadn't even ended when I blew my load all over myself. When I did, my sister looked over at me and laughed. "You sure made a mess, perv." She grabbed a handful of tissues and threw them at me. "Clean yourself up."