What is the legal ruling on a brother insulting his brother with abusive language such as "donkey," "despicable," "mentally ill," and "swindler"?

Cursing and slandering are forbidden and contrary to the ethics of a believer. If done unjustly to a biological brother, the sin is even greater, because a brother is a kin whom we are commanded to maintain ties with.

The offending brother should be advised to repent, treat his brother with kindness, and seek his forgiveness for the harm he has caused him. The other brother should be forbearing and not retaliate in kind, seeking the pleasure of Allah, as Allah the Almighty says: "And not equal are the good deed and the bad. 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."