Is slapping a sister who insults her father a sin?

What the sister did by insulting her father is an act of disobedience (uquq) and a great sin. Your advising her and slapping her is considered a change of an evil (munkar), but it is more appropriate for the father to handle that. Striking is not permissible for changing an evil or for discipline except under certain conditions, including avoiding the face, ensuring no greater harm results, and achieving the desired benefit from it. The one who changes with his hand should have authority and prestige, or seek help from someone who does. As for the mother, it is not permissible for her to help her daughter in insulting her father. Honoring parents is obligatory, regardless of what may emanate from them.