What is the ruling of Islamic law regarding dealing with a husband's mother who harms her son's wife with false accusations and seeks to separate her from her husband and children?

Trials should be met with patience and countered with what is better, and the husband should be thanked for his stance. The husband must advise his mother to stop slandering, as it is one of the greatest major sins, and he must advise his siblings not to condone her injustice. There is no obedience to a creature in disobedience to the Creator.