Is it permissible for me to keep the money that exceeds the costs of my father's treatment, or must it be returned to the donor, and is this money considered my father's property, entitling my siblings to claim it?

Regarding the funds exceeding the patient's needs, reference should be made to the donor's intention: If the donation was intended as charity (sadaqah), then the money is the patient's property, and he may dispose of it as he wishes. However, if the donor intended the money solely for treatment, or if it was given from his zakat, then the patient is not permitted to take the excess funds. Instead, he should take only what is sufficient for him and return the surplus to the donor. If it is impossible to identify the donor or obtain his permission, the money should be used for a purpose similar to that for which it was originally donated.