This happened about a year and a half ago. My wife has two sisters both younger than she is. They are all very close but the older one of the two younger sisters was closer to our family at the time. Our families kinda grew up together. So I never really thought of her in a sexual way. Until she turned 20. We went swimming and I saw her figure really for the first time. She’s a taller brunette big tits, tiny waist and a nice round juicy ass. But I never made a move until a year or so later. I decided that I was going to start groping her to see how she felt about me touching her. I started small. Back of the hand on her ass as I passes. Lingering hand on her lower back when we hugged. Then palm on her asscheek as I walked past. Then a full on ass grab during a hug. She knew what I was doing. Never said anything but she kept putting herself in positions to be touched.

One night she was spending the night and had on loose basketball shorts and a t-shirt no bra. My wife went to bed and I stayed up to watch a movie. My SIL came out of the guest bedroom to get some water. I stand up and follow her to the kitchen. I stand behind her and touch her stomach. She says, “what’re you doing?” As she presses her ass back into my crotch. I said, “I think you know what I’m doing.” She turns around to face me and puts her water cup on the counter. I slide my hands down her pants to feel her soft naked ass for the first time and I pull her crotch into my already rock hard cock. And then she kisses me. “You’re so big” she says.

We make out for awhile. I fondle her breasts and pinch her hard and sensitive nipples. She tells me she wants me to bite them, hard. So I do. We make our way back to the guest bedroom and we fuck for hours. This was the start of a very pleasurable relationship between her and I.