When is a man's marriage to another woman considered an injustice to his first wife, and what must he do so as not to wrong her?

A man marrying more than one wife is permissible, provided there is justice, and it is not an injustice to the first wife. Whoever claims otherwise is either objecting to the حكم of Allah or is ignorant. It is forbidden for the first wife to disparage the second wife and to slander her. It is even more forbidden to use verses from the Quran as evidence and apply them out of context. We advise the husband to be gentle with his wife and to be mindful of her jealousy, but he should not remain silent about her actions toward the other wife, such as slandering and backbiting. Rather, he should gently and kindly advise her about the impermissibility of these actions, because they are an obscenity that contradicts good morals. If he does not stop her from this evil, he is sinful and neglects his responsibility.