Over the summer, my wife's cousin got married in a beach town. Our side of the family decided to rent a big beach house for the weekend. There was my wife and I, my parents, her parents, and her two brothers and their wives. Her youngest brother is married to a girl who is 12 years younger than me. Prior to being with him, she was married to one of his good friends (there's a clue to her makeup). One night, we'd all gone out for dinner and drinks. Got back to the house and played some card games. Time goes on, one brother and his wife go to bed, then the other brother goes up. My wife eventually goes to bed, leaving me, my SIL, and my father in law. After he falls asleep in his chair, her and I start making faces at each other like idiot school kids. We decided to have one last drink on the porch. We go out and sit and chat for a while, right under the bedroom my wife and I were in. It was a little chilly, so she's snuggled up next to me. She brings up how it feels being the "outsider " in the family, and how they don't really accept her because of how they met. I kind of brush it off, because the family is good to me. But she moves closer, puts a hand high on my thigh and whispers "I guess once a cheater, always a cheater." I instinctively lean in and we start to kiss. Our hands start to wander, and pretty soon her hand is up my shorts stroking me, and mine is down her pants feeling how incredibly wet she is. Neither of us could take it anymore, and finally she stand up and goes to the porch railing. I come up behind her, slide her jeans and thong down, my cock in my hand and rub it up against her as if to ask "Are you sure?" As soon as I did, she wiggled her little ass back into me, working the head of my cock just inside her. I couldn't believe how incredibly tight she was! We fucked right there on the porch, my hand over her mouth to keep the noise down. She turned and told me I couldn't cum in her, and I told her she better get on her knees. She smiled and did just that, and she barely had to put her mouth on my cock before I exploded. She took every drop in her mouth, kissed the tip, stood up,pulled her pants on, and just went inside and up to bed. I stood there for a few seconds before I went in, still trying to figure out if that actually happened. We haven't said a word about it since, and I'm 99% sure no one in the family has any idea.