This story sounds kind of familiar.
I grew up with a brother two years younger than me and we shared a bedroom, and I also had 2 sisters. We had 1 bathroom for the 6 of us, and my mom didn't work so she was home just about all the time. We had a ranch style one-floor house, so my bedroom was pretty much near everything else in the house.
Most of my jacking off took place in bed after I felt my brother had fallen asleep. I'd be so horny and so erect that I could hardly stand it. Then, one night when he was 12 I heard him say in the dark room, "What are you doing??" (I felt he was almost old enough, so I showed him.)
Anonymous Big Bro