My Daughter?

I just found out that my daughter has been sleeping with my husband's brother. I've been trying to muster up the courage to confront either of them about it, but I just can't. What's a good way to subtly ease into talking to my daughter about this? I was thinking we could start talking about boys at first, then go on to the serious business, but I'd like your suggestions. My daughter is 14 and this is obviously not right. Her father doesn't know about it yet, and I'd rather preserve his relationship with his brother. Help?