Do we sin by not visiting the maternal uncle's house, or by the mother cutting off her relationship with her brother's wife and her daughter, or by not returning greetings, knowing that there are strong reasons for these actions, represented by the maternal aunt's past harm to the girls, and the repeated abuse of their mother, in addition to the maternal uncle's daughter's immoral and dangerous behaviors that led to the father's intervention to prevent them from visiting the maternal uncle's house?

Maintaining kinship ties is obligatory, and abandoning them is a sin. The questioner and their family must maintain ties with the maternal uncle and try to advise him. If he responds, then that is good. But if he and his family persist in disobedience, then cutting ties with them is their due, along with praying for their guidance. If the uncle's family insists on disobedience, then it is obligatory to shun them in obedience to Allah and in obedience to the father, if there is benefit in that. However, if they repent, then it is not permissible to obey the father in shunning them, because there is no obedience to a creature in disobedience to the Creator.