A week ago, I 37 m was helping my wife 32 move her sister 23 out of her college apartment. As soon as I got there, I knew her sister was horny. She hugged me slowly and brushed her ass against my dick. I suppose this wasn't the best day to not wear underwear. As we moved, my half hard dick continued to flop around in my shorts and I could see her looking at it and smiling at me. Eventually, my wife left with a full car while I drove her sister back home. The drive is about 2 hours from her college. During the drive, we began to talk about personal pages people create to make money. She said she tried one for a month and made some money. I was intrigued and asked to see what she would post (she claimed it was only feet pics). As she was showing me her feet pics, she accidently scrolled to a full nude pic. It was amazing... legs spread spreading her tight wet pussy while drooling on her tits. She became embarrassed and I assured her that she had nothing to worry about. That's when she slid closer and asked to see me since I saw her. I was fully erect at this point and pulled my dick out of the leg of my shorts. She immediately began to suck me and gag/drool on my cock. I had to fuck her. I pulled off to a gas station and we went in the bathroom with an outdoor entrance. I bent her over the sink after eating her and tucked her as she squirted and moaned. I came deep inside her and she loved it. We flirted a little bit more on the way home and promised each other we wouldn't say anything to my wife. I have felt a little guilty all week but co tinge to get off to this memory a few times a day.