What is the Sharia ruling on a husband who sends people to harass his wife by phone, then sends his brother to physically assault her, with the aim of proving her bad character so that he can marry another woman?

The husband should have known that the default assumption for Muslims is innocence. One is forbidden from harboring ill suspicions, spying, and seeking out faults, based on Allah's saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy." And the Prophet (peace be upon him) said: "Do not spy, and do not eavesdrop." This is emphasized in the case of one's wife, as a man is forbidden from returning to his family at night "to suspect them or to seek out their faults."

As for the brother's action, it is cooperation in what is impermissible, in addition to two forbidden acts: The first: being alone with a non-mahram woman, for the Prophet (peace be upon him) said: "No man should be alone with a woman unless she is accompanied by a mahram." The second: holding her hand, based on the Prophet's saying (peace be upon him): "For one of you to be stabbed in the head with an iron needle is better for him than to touch a woman who is not lawful for him."