Is it permissible to consider the money paid for bringing a domestic worker for my elderly mother as zakat, given that my siblings cannot share in the costs, and I already bear her monthly expenses?

The cost of recruiting a domestic worker cannot be counted as part of Zakat (definition). If the recruitment of the worker exceeds the mother's essential needs, then it is not permissible to pay Zakat for it. However, if it falls within the mother's essential needs, then it is among the expenses obligatory upon her well-off children. This expense should be distributed among them according to their financial capacity, not per capita or according to inheritance shares. If a son is wealthier than his siblings, he bears the larger share of the expense. Since this expense is obligatory upon him, he is not permitted to pay Zakat for it.