Mine came onto me several years before I married her sister. I was really in love with her sister, but did my SIL for the great sex.... almost 10 years, once/twice a month! As I look back my SIL would have made a good wife. Great housewife and awesome sex. But high upkeep!
I did mine back when I was in the army,she was my brothers third wife. He thought she was just all into him and a classy chic. While on leave I was visiting them,one day he had to leave to go do something at work and while he was gone one thing just led to another between me and her. I ended up fucking her bent over the couch in the living room,after that a few days before I had to go me and her got it on again while he was at work. He still knows nothing about it. They divorced a few years later,he caught her getting fucked by a co-worker. It didn't bother me that I fucked her,my brother was two faced as the day is long and back stabbing and of course she was no better loll. Karen was nothing but a sl*t. My situation happen in the early 1990's. So does your sisinlaw hubby know you two are fucking ?