What is the ruling on a husband's mother participating in backbiting her son's wife based on the mother's incitement, and what is the expiation for this sin?

You must cancel gatherings where people are spoken ill of, and resolve not to accept the continuation of backbiting and slander, even if what is said is true. This is because a person will need a single good deed on the Day of Judgment, and backbiting and slander are major sins. You must repent from this sin by seeking forgiveness for the person you backbit, praying for blessings upon them, and mentioning their good qualities. You must also defend against backbiting, and if you cannot, then you must leave the gathering where Allah is being disobeyed.