Is it obligatory for the husband's male siblings to help financially support their mother, given that they do not allocate any money for her, relying instead on their brother?

If the mother and sister have their own money, their expenses are covered from it. Otherwise, the mother's maintenance is obligatory upon her children, according to the consensus of scholars. The same applies to the sister, according to the preponderant opinion. The author of Kashshaf al-Qina' said: "He is obligated to provide maintenance for everyone he inherits from, whether by fixed share or by agnatic kinship."