You should be able to claim her as your dependent under the Qualifying Relative rules if she meets all the requirements. You will need to contact the state Medicaid agency to find out if claiming her as a dependent will affect her Medicaid eligibility.
To be a Qualifying Relative -
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,200 (social security does not count) in 2019
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.
She "might" be a qualified relative for the $500 credit.
IRS interview to help determine who can be claimed: