I agree with those who say that you need to tell her or something. Included or not, you are involved by knowing. They have kids and better sorry now than later doomed. You should just convince her to tell him. What if your husband was cheating, and his brother knew it, perhaps even his friends, and even JJB, and there you are, all clueless about it thinking everything is good.. It might be difficult now and I understand you don't want stick your nose in it or be part of it, but then you indirectly do support cheating. This is why they say cheating hurts also people around, like you. Don't to tell him but do at least try making her see what's she doing.