If she has gross income in 2024 of less than $5,050 you should be able to claim her under the Qualifying Relative rules if she meets all the requirements under the rules.
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 $5,050 (social security does not count) in 2024
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 with the following exception -
You can claim a person as a dependent who files a joint return if that person and that person’s spouse file the joint return only to claim a refund of income tax withheld or estimated tax paid.
It depends. Dependents must make under $5,050 in 2024, so she meets that test. As @DoninGA points out, there are other tests to be a qualifying relative, so review the list to ensure she meets all requirements.
As far as filing a return, she isn't required to file a federal return. If she had any federal income taxes withheld, she can choose file in order to get a refund. Also, check your state's instructions to see if she needs to file a tax return. Some states require tax returns to be filed, even if you don't make enough to be required to file a federal tax return.