Is it permissible for a divorced woman who is caring for her disabled sister to spend from her sister's money on herself and her son in return for her care?

If a mentally disabled sister is unable to manage her own affairs, her legal guardian is first her father, then her paternal grandfather, then their appointed executor, and then the judge. The disabled sister's sister is not entitled to dispose of her money. If she undertakes her care, an agreement must be reached with her legal guardian regarding payment for her services. She is not permitted to take money for herself or her children under the pretext of providing care. She must cease such actions and hand over her sister's money to her guardian, or, if no guardian exists, refer the matter to a judge. For past actions, she is entitled to a fair wage (ujrat al-mithl) determined by a judge or by experts.