How can you get disability checks for someone who is permanently incapacitated?

There is no POA. There is a health care proxy with the patient's sister as decision maker. The patient's daughter is cashing the patient's disability checks for her own personal use instead of the intended beneficiary. The patient's sister wants the disability checks to be issued to her for the patient's sole use. Since she is already making all decisions related to her medical care, how does she get control of her financial care? She does not have money for a lawyer.