I have had experiences with this one sister-in-law before, but none quite like this.
It was just a few weeks before Christmas and one of my wife's sisters was visiting for the weekend. I have had sex with this sister-in-law before, as my wife and I occasionaly would swing with this sister and her former husband over ten years ago. In fact, I often masturbate while fantasising about her and watching her husband have sex with my wife.
But this Friday night we had been out, and my wife decided to call it a night. So, her sister and I stayed up a little later, and started massaging each others feet while sitting in chairs directly opposite each other. I grabbed her foot and placed it firmly in my groin. I was just wearing thin pyjamas and she could clearly tell that I had a rousing hard-on going.
I let go of her foot and she started moving it in little circles on my dick, rubbing through my pj's. It took all of about thirty seconds for me to cum a geyser in my pants. She giggled and kept rubbing until I couldn't take it anymore. I went into the bathroom to clean up. When I came back out in the living room, she just smiled and said, 'I think I'll go to bed.' I will remember this night for a long time. I hope to have more stories about masturbating with her in the near future.