This began when my wife, her brother, sister-in-law, and I went to a drive-in porno movie together. After the show we discussed what to do next. My sister-in-law suggested going back to their house and having an orgy. We got naked together and each couple masturbated their own partner while the others watched.
On another occasion my wife, her brother, and her sister-in-law again got naked together and my wife masturbated her brother while his wife watched. Sometime later on my sister was visiting us. My wife was sleeping and I felt very horny and walked out to the living room where my sister was on the couch. I sat down next to her and told her about my wife's experience with her brother and sister-in-law and asked my sister if I could show her my penis. She readily said 'yes!'. I stood up, unfastened my robe and sat down next to her naked. I had a very stiff and full hard erection.
As I sat there with my sister looking at my erect penis and commenting on how nice it was and how she liked seeing it, I was slowly stroking it with my hand. The more my sister watched me and the more she talked about how she liked my hard penis, the more I stroked it. Finally I told her I needed to masturbate to orgasm. We must have spent at least an hour talking about sex while she watched me stroke my penis. I couldn't stand it any longer and; I don't know why, but I left her and went to the bedroom and masturbated to orgasm there!
I can't believe I didn't finish masturbating and had an orgasm with my sister watching me! I tried and tried to get her to let me be naked with her after that but she wouldn't do it again. She now lives far away but I would love to be naked and have her watch me masturbate to orgasm!