Is the questioner obligated to spend a portion of her salary on her parents, or is that obligatory for her brothers, knowing that her financial situation is weak and she has three male siblings?

Children are not obligated to spend on their parents if the parents are well-off, except as a voluntary donation. However, if the parents are needy and poor, then it is obligatory for the children to spend on them appropriately, as long as they are able. Spending is not obligatory if nothing remains from the child's money after fulfilling their essential needs. If there is money left over and the siblings are more financially able, then the preponderant opinion is that the expenditure should be distributed among the children according to each one's financial capacity. Children must honor their parents and treat them kindly to the best of their ability, for the rights of parents are great.