Fiction- riding girlfriends sisters

I have had 3 long term girlfriends in my life, and I have slept with each of their sisters while we were going out. Girlfriends never found out about any of them.

1st, my high school gf, we had dated about 2 years at the time, her sister was 5 years older than us, and in college at the time. Gf worked late some evenings, and I would regularly go to her house and wait for her to get there, usually 30 minutes or so. Her parents were always going out in the evenings. I was a real dick in high school, I would sit around her parents house and jack off, snoop thru shit, whatever. I proceeded to snoop thru her sisters room this time thinking I was alone in the house. Much to my surprise, just as I open her dresser drawer, her sister walks out of her bathroom, into the bedroom. I just stare at her, no idea what to do, after a minute (really probably only a few seconds), I turn to leave without saying a word. As I reach the door, she says “is this what you were looking for?” I turn just in time to catch the sports bra she was wearing, and has now taken off and tossed to me. Needless to say, my horny teenager brain took over and all I could do again was stare at her now bare tits, which were nice. She came over towards me, pulled me back into the room, and pushed me on the bed, and proceeded to give me an incredible blowjob, before she rode me. Anyways, we have sex, finish, clean ourselves up, then act like it never happened. She doesn’t say anything other than, “you better get out to the living room, Lisa will be home any minute. “ it never happened again, and we never really ever talked about it. GF and I dated another 2 years before we finally just grew apart.