How can one act concerning a dispute between the wife and the brother's wife, the cause of which is a lack of greeting, and which has affected family relations and caused the parents to boycott the questioner because they think he is siding with his wife against his brother?

Reconciling disputes is among the best righteous deeds, while discord shaves away religion. Therefore, the questioner must intervene to resolve the disagreement between his wife and his brother's wife, and obeying his mother in this matter is obligatory, because what she calls him to is a good deed, indeed, one of the best good deeds.