Am I allowed to ask my brother's wife to change her disrespectful behavior, and what should I do if she doesn't respond, and should I greet her?

A kind word and good treatment unlock hearts and remove their coverings, reforming souls and restoring affection. A kind word and overlooking an offense make the offender apologize for their transgression or stop persisting in it. One should avoid people who deal harshly, confrontationally, and defiantly. We should sow kind words, reassurance, and smiles, and make them feel forgiveness and affection. Allah Almighty said: "Repel [evil] with [that] which is better, and thereupon the one whom between you and him is enmity will become as though he was a devoted friend. But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]." And Allah Almighty said: "Take [O Muhammad] the forgiveness, [enjoin] what is good, and turn away from the ignorant." Reprimand, disparagement, and criticism yield counterproductive results. Humility and gentleness remove barriers and prevent aggression and dispute.