I have seen my brother jerk off several times. I am 2 years younger and I realize that my (straight) brother must have really gotten off on me seeing him because of how often it happened.
The way our bedroom was set up, with a stereo cabinet thing between our beds, against the wall at the head of the beds, we could see each other in bed without seeing each others' faces. Somehow, this made it feel less "intimate".
This might be gross to some, but it was hot to me. My brother has a thick dick (family trait) and was a bit of a bodybuilder since age 15, so he was perfect eye-candy. I loved watching him please himself and was never uncomfortable with it, except once when he asked if I wanted to suck him, get sucked by him, get fucked, fuck him... in that order. I couldn't make myself do it, but sometimes I regret that.
In our later teens, we used to stay up late together and watch HBO movies, wearing only our underwear. Our hands were often in our (own) underwear, playing with ourselves and I was often fully erect. He would sometimes break out into a full jerk off, usually with the band of his underwear across his knuckles or cupping his nuts, but I never got the courage to


as blatantly, there beside him. One night, when this had been going on for the better part of a whole movie, I had to get off, so I stood with my hand shoved down my underwear and asked him if he cared if I turned the movie off. He said "nah". My plan was to sit there in the dark and jerk off with him, but after I turned off the t.v. and sat back down on the opposite end of the sofa from him and started jerking off, it took him about 10 seconds before he got up and walked over, knelt of the floor and started sucking me off. I came so damn fast and I tried to hold his head down on my dick to make him eat my cum (I don't know why), but he was grossed out and easily lifted his head away. Then he jerked off on the floor right there in front of me in the dark.
So, yes, I've seen my brother jerk off.