Does the justification for the sudden change in the older brother's behavior towards his parents and siblings—which includes mistreatment and abusive language, despite him being kind to others—stem from the possibility that he has been subjected to black magic by his wife, and what is the solution to this situation?

Allah Almighty has made it obligatory for children to honor their parents and has forbidden their disobedience. What the brother is doing is a reprehensible act and a grave sin. He must be advised and reminded of the rights of parents.

It is not permissible to accuse others of sorcery, as the Prophet, peace and blessings be upon him, warned against suspicion.

One must seek the reasons behind the brother's behavior. If there are no apparent reasons, then he should be presented to a trustworthy raqi (exorcist) to clarify his condition. If it becomes clear that he is bewitched, then one must consistently apply what removes his sorcery from the Quran and Sunnah.

In all cases, dealing with him requires deliberation and wisdom, for he is sick and needs gentleness to deliver the appropriate treatment.