What is the ruling on a wife cursing her mother-in-law?

It is forbidden for a wife to curse her mother-in-law, because cursing a specific Muslim is unanimously Haram (forbidden). The Prophet, peace and blessings be upon him, said: "The believer is not a frequent curser, nor an obscene speaker, nor an abusive one," and "Whoever curses a believer, it is as if he has killed him." Furthermore, if a curse finds no legitimate target, it returns upon its utterer.