My sister and I have never spoken of this since. It would be embarrassing if she reads this site!
I have been caught masturbating a couple of times. One time was by my sister. I was usually really cautious about it. I tried to do it when everyone was asleep, or, even better, when I was alone. Part of the reason was that I didn't have a lock on my door when I was a teen.
This particular day, when I was 15, I couldn't wait! It was Sunday, and I was so terribly horny; I needed to play. My parents were watching TV downstairs and my sister was in her room, right across the hall from me.
I was feeling brave, so I took off all my clothes. I had a fairly large mirror hanging behind my door, and I like to masturbate while watching myself. Things were going fine! I could hear my sister, 13, playing with our dog in her room. She usually didn't bug me, so I wasn't too nervous. Suddenly, the door crashed open and in ran the dog! I hadn't latched my door properly! I had no time at all to hide or cover up.
The dog jumped on me and my sister stopped in shock. I bent over to push the dog away, and also to try to cover myself. I heard my sister say 'I'm sorry!' and she too bent down to pick up the dog.
She grabbed the dog, and then pushed the door shut behind her. I stood leaning up against my bed (it was a tall bed) and covered myself with my hands. I kind of hunched over with my arms covering as much of me as possible. She giggled a little and said 'let's see!'
I wasn't in a very strong bargaining position, but I argued anyway. She swore she wouldn't tell anyone, and she just really wanted just to check me out. Finally she said she wasn't leaving until I showed her. I gave in.
I was just a skinny little nerd, and I wasn't really big. She got on her knees and looked me over really close; she even put her hand on me. She asked me to show her how I masturbated. I was so aroused, I could hardly stand it! I showed her. I ejaculated so hard that I got cum in my hair. My sister screamed with surprise. I've rarely cum that hard. I thought my parents would come upstairs to see what was going on! They didn't, and thank God!
We never did anything like that again. Unfortunately, I never saw her naked since she was nine or so. She kept her word and never told anyone about it, although looking back now, I don't think it would have mattered at all.