My name is Joe. My wife, Liz, and I often take our kids on vacations with her sister, Amy, and their kids. We went to Florida last year and rented a beach house. It was small with only two bedrooms, but we figured the kids could sleep on the couch and floor as we would be on the beach most of the time anyway. The first night there, we got in late and piled our stuff in the place. It was smaller than we thought. We each took a room. While the girls got the kids ready for bed, my brother-in-law, Barry, and I decided to go find a local watering hole and have a drink since we were tired from the drive. One thing led to another, and we got a little drunk. We stopped off on the way home and got a six pack. When we got to the beach house, we saw that the kids were all over the floor and sleeping soundly. The place had a nice porch including a hammock, so we decided to relax there and have a couple more beers. We talked for a while, and then I noticed that my brother-in-law had passed out or fallen asleep in the hammock. I decided to leave him and go to bed. It was pitch black, and I had to be careful to not step on any of the kids. I slowly picked my way to my and my wife’s bedroom. I quietly stripped and got into bed naked. I snuggled up to Liz and found that she was naked too. This was unusual, but I liked it and started to let my hands roam. She purred in response, and it didn't take long before we were having some very quiet but very satisfying sex. After about fifteen minutes, I was just starting to climax, and I was pretty sure she was, too. Then she suddenly started calling me Barry. I said, "What did you call me? Liz? Is this Liz?" Then I heard, "Oh, my God. Is that you, Joe?" I realized that I was in the wrong room and the wrong bed with the wrong sister. I couldn't stop because I was just starting to climax, but amazingly she didn't stop either. It took us another minute or two to finish, and then we just laid there with me on top and whispered back and forth about what had just happened. Apparently, after Barry and I left, the girls had decided to change rooms never dreaming that Barry and I would get confused. I was apologizing, and she was telling me it was okay, but I was still on top of her. We were agreeing that we would keep the whole thing as our little secret when I started getting excited again. She felt it and started to move. We ended up quietly doing it a second time. We have never done it again, but there is now a ton of electricity between us, and she never misses a chance to give me a hug or to rub my shoulders.