Firs off, if she has gross income of $4,200 or more you cannot claim her as a dependent (social security does not count). If she only receives SS income then you have to determine if you provide over one-half of her support for the year. Use this IRS website for determining support - https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
You claiming her as dependent should have no affect on her tax return since she would receive the same Standard Deduction either way but she must meet all the requirements under the Qualifying Relative rules for being a dependent.
However, if she receives any state aid, such as Medicaid, then you claiming her may affect her eligibility for state aid. If you are able to claim her as a dependent and she is filing a tax return then she MUST indicate on her return that she can be claimed as a dependent.
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.
