Is insulting the other's mother jokingly considered a sin?

Cursing the mother of one's wife or any other Muslim is not permissible, even in jest, based on the Prophet's (peace be upon him) saying: "The believer is not one who curses much, nor one who is obscene, nor one who is abusive." And his saying: "A Muslim is one from whose tongue and hand the Muslims are safe." And "Abusing a Muslim is Fusuq (disobedience)."

This action is either forbidden backbiting if the descriptions are true, or slander if the words are false, and both are prohibited. The Prophet (peace be upon him) would joke but would only speak the truth.

Moreover, this act may lead to cursing one's own parents, for the Prophet (peace be upon him) said: "Indeed, among the greatest major sins is for a man to curse his parents." It was asked: "O Messenger of Allah, and how does a man curse his parents?" He replied: "He curses a man's father, so that man curses his father in return, and he curses his mother, so that man curses his mother in return."