How do I confront my sister - who cheated on her husband with the guy I'm currently dating. Read below please.

If you've read my previous question, you know that I have been dating a wonderful guy that happened to have a one night stand with my sister a few months before we met. (see other question) I've since decided to keep dating the man and move on from the issue. However, I feel like I need to say something to my sister. At least let her know that I know. I'd hate for her to think that I don't know and that the guy I'm dating is a jerk and he didn't have the decency to tell me about their encounter. That could cause more problems down the line. But I love my sister and the same way that I handled the situation with the boyfriend I will handle it with her. The thing is, she's married. I won't say anything to the hubby. But should she? Again. More problems could arise. Her hubby would be sohurt! I'm dating the man his wife cheated with. Would telling her that my boyfriend revealed their secret open up a can of worms? Pandora's Box? A can of whoop? What do you think? Thanks everyone!