One day about a year ago, my sister (who was 16 at the time) barged into the bathroom as I was sitting on the toilet jerking off. She saw what I was doing and freaked. I got really scared that she was going to tell my parents that I was masturbating (they're the crazy you-can-never-masturbate-because-the-Bible-says-so type). She in fact started telling me she would tell Mom. I pleaded with her not to tell. I got her to promise not to tell, but she had one stipulation: I had to jack off in front of all of our siblings. That meant I had to jack off in front of her, my 15 year old brother, my 14 year old sister, and my 13 year old brother and sister (twins). I agreed to it just because I feared my mom that much.
She marched me downstairs to the family room where all the other kids were watching TV. As I walked in, my 15 year old brother, Dave, noticed my boner in my shorts and started teasing me. Everyone laughed, as even the 13 year olds knew about boners. My sister who caught me, Patricia, started to tell everyone about how she caught me and that my punishment would be to do the deed in front of everyone. My brothers thought this was hilarious and started laughing hysterically.
Patricia made me take off my shorts and boxers and stand in front of my siblings jacking off. After about five minutes of pounding my meat, I began to cum and aimed for Patricia who had found a seat right in front of me. Some hit her, some just fell to the floor. She quickly left. I think she jilled off but I can't be sure. Next to leave was Dave who had an obvious hardon and later admitted that he went to jack off in his room. I then left and made my youngest siblings clean up.
Ever since then I haven't been bashful with my masturbation or nudity. Whenever my parents weren't home, I'd never close the door when using the bathroom, letting my siblings stare at my cock as I pee, shower, take a dump, and even shave naked. I never closed my door when I changed. Most importantly, my siblings saw all the jacking off they wanted. When I jerked off in my room or bathroom like old times, my door was never closed. I also moved to public spaces and jacked off in front of everyone while doing everyday things like watching TV. Sometimes I'd even watch porn with others around, including my sisters. It was all great fun and really exhilerating with an audience.
Eventually my brothers decided to join me. So sometimes all three of us would be jacking as our sisters watched. The girls never got naked for us, but we never actually wanted them to. We just wanted them to watch.
I haven't really done this since I've turned 18, but my brothers definitely still do.