What is the religious ruling on a woman causing discord by relaying negative truths about her husband's family to his brother's wife, after having suffered from them and forgiven them, only to discover that they are still speaking ill of her?

Affection and harmony should prevail between you and your in-laws due to your kinship. Any shortcomings on their part are a trial that should be met with patience and forgiveness, for the Prophet, peace and blessings be upon him, said: "Calamity continues to afflict a believer, male or female, in their body, wealth, and children, until they meet Allah with no sin remaining upon them." He also said: "Allah does not increase a servant who pardons except in honor." As for the woman relaying their conversations about you, that is forbidden backbiting. However, informing her of their faults by way of advice and warning is permissible, as long as you do not mention specific names to avoid discord.