<human>: Alright so throw away for reasons. Basically I live with my cousin, her fiancé and their 2 kids. She was caught by her fiancee texting inappropriate things to her new male co worker of a week. Not just sexual things but more lovey dove things as well. When she was caught he was devastated, made her tell her mom, etc. He stayed. Sadly I knew the whole time about it, and knew about the time previous to that that she did the same thing with another guy. So I feel like a jerk to start. 

But! Now she talks to me all the time about how she feels about this guy, because she's still talking to him. Even going behind the fiancees back to hang out with him. They've gone for lunch, met at a park, etc. She even came to me and told me about how "something" happened between them. Not sex though. 

I have already had the "Don't stay together for the kids if your just going to be unhappy" talk with her. And she really didn't listen. But I can no longer take knowing all the dirty secrets about her affair. I like her fiancee, he's a good man and doesn't deserve this at all. Especially when he's trying to work this out and she just isn't trying at all. 

I really really want to tell him. But I would be the person that "wrecked" the relationship in a sense. But he deserves to know and she deserves to learn a lesson. I don't know. I just don't want to have all this information in my head anymore. 

What do I do?!

Give me the gist of the text in a nutshell
<bot>: Cousin is going behind fiancees back to talk and do stuff with a coworker, I know all about it and I don't want to know anymore. Thinking about ratting out cousin to her fiancé, thoughts?