My wife caught me

I'm married have 2 children and a beautiful wife, I was really horny my wife was pregnant and in the last month of her pregnancy. Sex was off the table and morning sickness ruled our lives. So, I was home alone, and my wife and son were out with her mother (widow) and should've been gone several more hours. Her older sister dropped by, and we started talking, her sister wasn't married and was between boyfriends. Well, things just led to us fucking and my wife had gotten and had returned early, her mother taking our son to the park to feed the ducks at the lake. He had been promised he could feed the ducks, my came walked in on me fucking her sister and told me to finish what I had started while she sat an watched us. Awkward would be a huge understatement, once finished she thanked her sister for fucking me and the sister replied anytime sis dressed and left. It turned out my wife had asked her sister to fuck me, her sister had mentioned she wasn't getting fucked and so my wife asked her to service me (her words). A short time later her mother arrived with our son and she of the arrangement made by my wife. Two weeks later our baby girl arrived, and her sister and mother moved in with us. I'm allowed to fuck her sister and will be as long as her sister is willing, and I'd overcome my shock and asked what about your mother. Expecting a shocked reaction of some type, instead she said she would ask her mother and I was again shocked. As yet I haven't been told she wants sex with me.