Is it a religious obligation for the daughter to spend her entire salary on her sick mother, given her wealthy brothers' refusal to contribute and her being the sole provider for her?

The maintenance of poor parents is obligatory upon their wealthy children, both male and female, and it is a form of kindness enjoined by Islamic law. The distribution of this maintenance among the children is subject to several opinions, including that it should be according to inheritance (the male receiving the share of two females), or equally, or according to financial capability. If the brothers fall short in providing maintenance, the obligation does not fall away from the well-off daughter, and she has the right to demand her brothers' shares. It is preferable for her to consider her maintenance of her mother as a reward, because honoring parents is a great door to Paradise. She should advise her brothers that maintenance is obligatory, and the mother has the right to sue anyone who refrains from spending.