Would a son be considered undutiful (ʻāq) or negligent if his mother asked him to treat her daughters as he would his wife and provide for their needs, given his financial capability, and he refused most of the time, claiming he had fulfilled his duty towards his father?

It is not obligatory to equalize the financial support (nafaqa) between the wife and sisters; for the wife's financial support is obligatory upon her husband, and takes precedence over the financial support of parents. As for the financial support of sisters, it is obligatory upon the father if they do not possess wealth, and it becomes obligatory upon the brother under certain conditions. If there are multiple affluent brothers, their financial support is incumbent upon all of them. The mother is permitted to take from her son's wealth in a reasonable manner. Raising one's voice at the mother is an act of disobedience (uquq) that necessitates repentance.