Is it permissible for a divorced woman who has been harmed by her former mother-in-law to warn the family of her former husband's brother's future fiancée against her former mother-in-law?

The basic principle concerning the honor of Muslims is its inviolability; therefore, backbiting is not permissible. Allah the Almighty says: "And do not backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful." (Al-Hujurat:12). No exception has been mentioned allowing backbiting the suitor's mother, so speaking ill of her remains under the original prohibition. Her ill-treatment of you does not necessarily mean she treats others similarly. One can repel evil with good. Allah the Almighty says: "And not equal are the good deed and the bad. Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend." (Fussilat:34). Indeed, showing kindness to one who has wronged you can transform enmity into affection.