What is the Shar'i (Islamic legal) conduct towards a sister's husband who abuses with words and slanders in a way that touches upon honor and reputation, and financially extorts the sister by having her imprisoned under the pretext of a trust receipt, then demands the payment of the amount, and is refusing to pay the amount considered disobedience to the mother, and is there sin entailed by these matters?

The marital bond is a relationship that must be honored. The Prophet, peace and blessings be upon him, enjoined good treatment of in-laws. What your sister's husband is doing—abusing, slandering, and blackmailing—contradicts these manners and is an ugly and forbidden act. We advise you first to be patient and pray for him, and for knowledgeable people to counsel him. If he does not desist, he should be threatened with reporting his actions to the authorities. If he still does not desist, then the threat must be carried out to stop him. It is not permissible to kill him, as he has not committed an act that warrants it, and the fundamental principle is the sanctity of a Muslim's blood. You are not obligated to obey your mother in paying the amount, but work to appease her and avoid her anger. Strive to prevent this man's deceit and to prove his falsehood.