Is it permissible to boycott a brother who is abusive and insulting towards his mother?

If a brother mistreats his mother and is insolent towards her, he is committing a grave sin. His sister must command him to be dutiful to his mother and forbid him from disobeying her. She should warn him of the consequences of disobedience in this world and the hereafter, and make him aware of his mother's rights over him.

If his sister believes that shunning him will deter him from disobedience, then she should shun him. But if she believes that maintaining ties with continuous advice is more beneficial for him, then she should not shun him. She should persist in advising him with wisdom and gentleness, hoping for his guidance and repentance, and disliking the sin, not the sinner.