Sister-in-Law Sex Addiction

For years my family would vacation at a resort in Minnesota. One year, my wife's youngest sister came with us. I had know her since she was 8 years old. She grew up the be a very attractive young woman. She was a full figured brunette, just like my wife. She had a killer body. Big bouncy bubble butt, and a pair of tits to die for. She had relationship problems. Mostly due to well documented sex drive. She came with us on this trip because she was going through yet another break up. Every morning the guys would go out fishing.
When we would come back in for lunch, she was always sunbathing on the little beach next to the dock. When she heard the boats coming up, she'd turn over, so we didn't get a full frontal view of her in her bikini. But that was fine by me, because she'd be showing off that fine butt of hers. One day as I got out off the boat, she caught me staring at her ass. She lifted her sun glasses, and said " I don't think your wife would want to hear about you staring at her little sister butt, now would she?" I just muttered something like " What are you even talking about?" That night at the campfire everyone hade been drinking. My Sister in law started talking to everyone about when I first started dating my wife, she was only 8 years old. She told everyone what a big crush she had on me. She said she thought I was nicest guy in the world, and always wished I was her boyfriend. My wife got up and hugged her. She told her " You'll find a guy like Scott someday. The next morning I was to hung over to go fishing. The girls were all going into town to shop and have lunch. I'd have the cabin to myself for a few hours. But at the last minute my Sister-in-Law decided to stay back. I heard her tell the other, " My sunburn is really bothering me". I waited about 29 minutes for everyone to leave, then I went out into the kitchen. Kari was doing the dishes. She had a long tee shirt on, and what looked like a bathing suit underneath. Her brunette hair was curly, and down to her waist. I started joking about how hung over I was. She laughed and said " You probably Don't remember much about last night." I said I remembered her talking about having a crush on me. She laughed and said " I really did have a crush on you when I was younger. Truth be told I probably still do a little. " I laughed and said " So your not made at me for staring at your ass the other day?" She turned around and said " I'm not mad at all. It's our little secret. " I then said " Kari, you know how much I love your sister ,right?" She was like " Sure I do, why are you saying that?" I said " Because I have had a crush on you the last few years." I continued " since you turned about 25, I know the problems you have been dealing with. But since you been hanging around our house, and seeing in a bikini by the beach, I've been secretly fantasizing about you." She turned around and faced me. " Would you like me to take off my tee shirt and parade around in front of you. I said " Why not, it's just your bathing suit. She walked past me into her bedroom. I was like, oh shit I took it to far, and now she's pissed. She walked back out into the kitchen. She wasn't wearing a bathing suit. She was in a white lace bra, and a tiny white thong. She looked at me and said "Is this better?" I was speechless. She walked right up to me and said " Scott, I've wanted to suck your cock since I was in junior high." I ran my hands all over her big ass as we kissed. Then she took my hand and walked me into her bedroom. She sucked me off for 15 to 20 minutes. Then I fucked her from behind until we both came. We made the most of the next 3 hours. She did things I've never done with my wife. We made a promise to never do this ever again and to never speak to each other or anyone else about it. And we never have.