Is it permissible for me to visit my relatives (my mother's family) despite my father's refusal, and would I be considered sinful if I visited them?

Grandparents, uncles, and aunts are considered kin (arham) whose ties must be maintained. It is not permissible for a father to order his son to sever ties of kinship, and he must not be obeyed in this matter, for the Prophet, peace and blessings be upon him, said: "There is no obedience to a creature in disobedience to the Creator." Furthermore, telephone calls alone are not sufficient to maintain kinship ties if one is able to visit them.