Is your sister collecting unemployment benefits? If not, what is her source of income? In the USA, if she qualifies for social services due to her low income status, she automatically is eligible for Medicaid. She does not need a diagnosis to qualify for Medicaid. Anyone with a low income that meets state eligibility guidelines can receive Medicaid.
Check with your local social services office in your county to determine her Medicaid eligibility...and apply for any other services that she is eligible for as well.
Also, she does not need to be your dependent to be eligible for social services and Medicaid. In fact, if she were your dependent and your income is high enough to be over the income guidelines for Medicaid eligibility, she could be denied Medicaid.