Would you tell someone they are being cheated on?

My bf and I are half broken up and half together. Living together but not seeing a future together and looking to see other people. We got to this point because he was cheating on me. His brother was giving him advice on how to cheat. I overheard a conversation when they were on the phone that his brother cheats on his wife and tells her he’s going on “guys night out” and changed his passcode so she doesn’t find out. Should I tell her we’ve both been cheated on by these brothers or mind my own business and move on?