He knows that his wife is not faithful to him. You should discuss with him about it in straight manner provided he is ready. You have to show your eagerness to support him for any extreme decision like divorce. However, if he is not keen to take your support then you have to leave him to fight his own battle after conveying him that you are withdrawing because he does not want your help. I also feel that apart from helping your brother, you seem to have some revengeful attitude towards your bhabhi. Check this feeling too.