Did anyone else do this with their spouse's sibling?

I've been married to Mark for five years now. I know he has a younger sister who lives in town, and last month we ran into her while out on a jog. She's... different from him, more outgoing, confident in herself. We ended up talking for hours that day at this café downtown, and there was something about her that just grabbed me. A few weeks later, after Mark left on a business trip, I reached out to her, she came over to my place, we had wine, and... things happened between us. It’s been happening regularly since then, when Mark is busy or gone for work trips. Can anyone else relate? Have you ever cheated with your spouse's sibling?