What is the ruling of the Shari'ah concerning a husband who allows his brother to speak ill of his wife and her deceased parents, and allows his mother to insult his wife, without taking any action regarding that?

It is not permissible for a husband to allow his brother, mother, or anyone else to insult or harm any Muslim without right, especially his wife, whom Allah has commanded him to treat with kindness. He must prevent this wrongdoing and forbid it gently and respectfully, especially when dealing with parents.