I don't know what it is about seeing your wife's identical twin sister naked, but it is a turn on. They look so much alike it is difficult to tell them apart. The main difference is their voices. About 10 years ago me and my brother in law tried to get them to swap partners, but they refused. They did however agree to strip and have masturbation sessions. We had one last night and it went down like this.
The twins put on a great strip show while me and my brother in law watched. Once they were totally naked we quickly pulled our clothes off. We started watching a nudie movie, but shut that off after about 10 minutes and started watching each other. Peggy, my wife's twin and I started kissing as Patty was kissing Peggy's husband. After everyone was extremely excited the girls sat side by side and put on a masturbation clinic for us. After they made themselves cum I started playing with Peggy's pussy. Occasionally I would glance over at Patty and could tell she was have a ball. Before I came the first time my wife came over and jacked me off with Peggy underneath me. I shot cum all over Peggy's pussy. I then watched Peggy jack her husband off and he shot his cum all over my wife's tits. Both women have bodies so hot. They are five-seven, around 135 with 38D tits and the most beautiful two asses you have ever seen. I usually spent more time playing with Peggy than I do my own wife because I play with her every night. There is no telling how many orgasms our wives had before we stopped playing. I came twice and was hard when we stopped. Sometimes me and my brother in law will jack each other off. That is not our favorite way to cum, but we do it to please our wives. Of course our favorite part is watching Patty and Peggy finger each other and get each other off. We get together almost every night, but our masturbation sessions are limited to twice a month and we all look forward to them. And when they end the four of us are covered in cum and pussy juice. You would think it would be more fun if the girls didn't look exactly alike, but I don't see how it could be any better. Watching the two of them cum is like seeing double.