Almost a step-sibling, but not quite. She was my step-aunt (step-mother's younger sister). She was only about two or three years older than me though. She was super hot -- like a young Jennifer Love Hewitt. We went to different colleges in different cities back in the day when AOL IM was becoming popular. So I would chat with her on there, and sometimes we'd talk after we'd come back from a night out drinking. One night she tells me that her fetish is guys talking dirty to her. So I jokingly told her something like I wished she lived closer so she could come over and suck my dick. She took off from there and wrote me some of the dirtiest things I could even imagine about what she wanted me to do to her and when and where, all the time telling me that she was fingering herself and cumming.
The next time I saw her in person we were at her apartment and it was kind of awkward. Eventually we started kissing and I suggested we go upstairs to her room. She said she didn't think it was a good idea. I whispered in her ear a few dirty things and within 30 seconds she was dragging me up the stairs and ripping off my clothes. It was really that easy. We spent a few weekends together in bed and I'm not sure if she told anyone else about it or not. Nobody in my family has ever mentioned it to me.
A few years later she came to the town where I was working. By that time we rarely talked. She brought some friends up with her so we didn't have any time to ourselves. One day they were all by the hotel pool when I stopped by and told her that she could use more suntan lotion. She told me I could put some on her back and I asked her quietly if she wanted me to rub it on her tits too. Again, that's all it took. She told her friends something ridiculous like she wanted to show me her hotel room and we ran up there and had a quick fuck.
We are both married now (to other people) and I'm tempted to see if she still responds the same way. Neither of our spouses know about our past so they wouldn't be suspicious if we were hanging out together. Maybe at the next family gathering I'll see if I can get her going again.