What is the Shar'i stance regarding the grandmother and the father who cooperate in harming the mother, and is maintaining kinship ties obligatory in this case?

We advise you to supplicate frequently to Allah to mend the situation and spread harmony and affection among your family members. You should also advise your father and grandmother with politeness and respect to cease their hurtful actions towards your mother, for the Prophet, peace and blessings be upon him, said: "Religion is sincere advice." If the advice does not work, continue to supplicate for them. You are not permitted to sever ties with them, as severing kinship ties is not permissible. It is not permissible to mistreat parents and grandparents; rather, it is obligatory to maintain ties with them, treat them kindly, and show them benevolence.