After I married

I'd to marry as I'd gotten my girlfriend pregnant, it wasn't against my will as I really love her. But it was quicker than I thought it would happen, I'd just turn 20 and my girlfriend was 18. No problem I would still go to university and live off campus with my wife and her mother 43 divorced. My wife is youngest of 2 and only one still living with her mother. As they already lived close to the university, it was thought it would best, we live with my mother-in-law (mil). After moving in, my wife went to visit her father, before he went overseas again, she was 6months pregnant and sex had become limited between us. She left on a Friday for 10days and that evening I was reading a history book getting caught up on the subject we were to study more fully the next week. When my mil came in in her bathrobe and asked me was I up to having some fun. I answered yeah always, and she then dropped her robe and was totally naked underneath. I was a little shocked, but quickly got over it and we had sex regularly my wife was away. On her return, sex still happened when my wife was out with her sister or my mother or my sister. My mil always had a reason to stay home, unless I wasn't home. No precautions are required as my mil had surgery and cannot have any more children ten or so years ago which led to her divorce as her ex-husband wanted a son and she couldn't now provide him a son. Otherwise, they were on good terms, her sexual appetite had somewhat increased since her daughter got pregnant. But it couldn't last I thought, and I was making the most of it while I could. My mil had other ideas and been talking to her daughter, about the lack of sex I was getting. Unknown to me she convinced her daughter, that she should provide me with sex while she was pregnant. My wife was convinced by her mother and the first I knew of the arrangement was her mother joining us in our bed. Even today 7months after the birth of our son, my mil still joins us for sex. It was strange at first, but now I expect to have a threesome regularly. Best of all I'm not cheating on my wife anymore as she is involved. A fine point but it still counts I believe.