He—may Allah Most High have mercy on him—was also asked:

About people among whom one man marries another’s sister, and the other marries his sister or daughter. Whenever one provides maintenance, the other does likewise; whenever one provides clothing, the other does likewise. The same applies in everything, including pleasing and angering: if one is pleased, so is the other, and if one provokes anger, the other does likewise. Is this permissible?

He replied:

Each husband must either keep his wife on honorable terms or release her with kindness. He may not make this conditional upon the other husband’s conduct. A woman has rights over her husband, and her rights are not forfeited because of wrongdoing by her father or brother. Allah Most High says:

“No bearer of burdens will bear the burden of another.”

If one of them wrongs his wife, her rights must be enforced against him. The other man may not wrong his own wife because she is the first man’s daughter. If each wrongs his wife because of the other’s wrongdoing, both deserve punishment, and each wife is entitled to demand her rights from her husband.

Even if this were stipulated in the marriage contract, it would be an invalid condition of the same kind as “exchange marriage,” whereby a man gives his sister or daughter in marriage on condition that the other man give him his daughter or sister in marriage. How much worse, then, if he gives her in marriage on condition that, if one treats his wife fairly, the other does likewise, but if one wrongs his wife, the other wrongs his own wife! This is forbidden by the consensus of the Muslims. Anyone who does this deserves a punishment that will deter him from such conduct.