My wife's sister came to stay at our house. I've been feeling a bit pent up lately, having no sex, and I was thinking that it was getting hard to masturbate, when suddenly, my sister-in-law started playing with my nipples. "Hey, do men's nipples feel good too?" she asked, flicking my sensitive nipples, licking and sucking them until my erection reached its peak. Even though my wife was there, I couldn't resist, so I had sex with my sister-in-law...