I'm going to call out my buddy for cheating in front of his wife
Why do in public?
His wife is an old friend of mine, and seeing her being cheated on rubs me the wrong way.
I understand you wanting to stick up for friends. However, getting into the middle of a friend couple's relationship can and will get messy. You will very likely lose both people as friends at the end of this. Let the wife knows behind close doors or give your brother an ultimatum.
I'm only doing it for justice.
Make sure you'll know what you are getting into and ready for the outcome.