Is it considered disobedience if a son refuses to fulfill all of his mother's financial requests for his brother and sister—despite him covering her personal needs and having previously helped them—especially since his brother and sister are capable of earning a living?

Obeying one's mother is obligatory in what is permissible, and she has the right to financial support, comfort, kindness, and maintaining ties of kinship. It is permissible for her to take from her son's money in a reasonable manner. As for siblings, there is no obligation to give them money, but it is recommended to help them. It is not permissible for the mother to take money to give to someone other than you. Hajj is obligatory to perform immediately, so there is no obedience to the mother in forsaking it to help a brother, because there is no obedience to a creature in disobeying the Creator.