Maybe you can start a dialogue with him about why he cheated and perhaps that can give you more guidance if you tell or not. I don't think there is any good reason/excuse to cheat but it would be fair to listen before you do anything.
Do you love your brother? Revealing someone else's dirty laundry is never a good idea. It is something they should work out as a couple not a family issue.
Find someone you trust who is not in the family - who preferably doesn't and won't know your family - and explain the full situation. You need outside counsel.
I would not do that unless you want you family to hate you. Especially your brother. If this bothers you so much talk to your brother about it or else.