stay out of it. No one is pefect
That's exactly how I feel. It's too late. I'm involved now. I think she's feeling really paranoid about it and she'll do whatever she can to either prove he is or isn't cheating.
Now I feel like I should at least just ask my brother if he is stepping out. We tell each other everything so he'd tell me the truth. If he is, then I'll know and I'll feel somewhat obligated to tell my sister in law, but I don't want to. I don't want to be responsible for anything that happens between them but, dammit...why did she have to bring me into this?!?