Is marrying the daughter of my mother's brother without the father's consent considered a sin?

It is advisable to strive to persuade the father to agree to the marriage with the girl and to seek assistance from those who have influence over him. If the father refuses the marriage due to his disagreement with the mother, then obedience to him is not obligatory, and the son will not be disobedient to him by disagreeing with him on this matter. However, despite that, it is not permissible to treat the father poorly; rather, one must be kind to him, dutiful to him, and obey him in what is good.