But the son is apparently respectful. The mom may be a great mother and It is entirely possible the daughter is an entitled brat. We don't know. Taking it at face value, since the son is respectful and under the assumption there has been no mistreatment from the mother's side,  the mom should not tolerate the daughter's abuse. I am not saying the mother should slap the girl. But make it clear her behavior is not acceptable and if the daughter cannot treat her mother with respect then she needs to find another place to live. The dad should grow a pair and back mom up.