I'm married now about 10 years. My wife's sister was a year younger than her and before we were married would often go out on nights out in our company. One night around Christmas we had been out and my now wife's mother was away for the nite so the plan was for me to stay the night with my wife at her house. So the 3 of us got home in taxi. We went into the living room and my wife wanted to go bed as she was a bit worse for wear. She went to bed and whispers for me to follow. So 5 mins later I make my excuse and head down to bed. We started fooling around and we are going at it full doggy style when the bedroom door opens and wife's sister is standing there. I panic as I'm embarrassed and my wife doesn't notice as she is quite drunk. My sis in law says sorry and closes the door quickly and leaves. Feeling embarrassed I stop and my wife falls asleep. So I go to leave as I am seriously red with embarrassment. I have to walk past the living room to leave and my sis in law is sitting there. She said sorry I didn't realise what u were doing. I said I didn't expect her to come into room. The next thing she said was I wish someone was doing that to me in a jealous of her sister way. I took an instant hard on as I always had a thing for her. She had smaller tits and had blond hair and she always looked like she was dirty. I said it was a pity she didn't have a bf as she could be getting it. She looked over and then down at my cock which was bulging through my jeans. She got up walked towards me and turned off the light in the room so only the hall light lit the room up . She then reached down and grabbed my cock and went down on her kneles and she looked up and said I wanted this for a while. She took my cock out and it was still wet from my wife's pussy. She licked the tip with her tongue while massaging my balls. I was close to blowing already. She then started to stroke it with her hand while twisting it all the while it her mouth was sliding up and down. I couldn't wait any longer to get at her tits. So I lifted her straight up from her knees so she could straddle me and I could finally get them tits in my mouth. Quickly that followed by her leaning over the sofa with her ass in the air begging me to enter her. At this stage my cock was bursting to come. I entered her and she instantly leaned back putting her arm around my neck to let me massage her tits again and I lasted about another 10 seconds before I had to withdraw. She then Finished me off with her mouth . I left shortly after and had many a wank since over it. My wife and I are married and we see the sis in law regularly. We have never mentioned it to each other and nothing else has ever happened. It's still no 1 in my wank tank memory