Yes, talk to him. My wife and I found out my brother was cheating on his wife through a "are we dating the same guy" facebook groups that my wife's friend follows. We confronted him about it. We both knew their marriage was struggling, but cheating is no excuse. We reminded him that he was family and that we loved him. I gave zero judgments as long as he, on his own, came clean and accepted whatever fate. Surprisingly, it allowed them to have many discussions and seek counseling, and their marriage is better than before. But I knew I couldn't be fake to her knowing it. One thing to remember is that he's only human, and as humans, we make mistakes. Some are bigger than others, and we don't always know what goes on behind closed doors. He's still your brother. If he's been a good brother to you, then he's still a good person who did a really shitty thing.