For you to be able to claim her as a dependent in 2019 she must have gross income of less than $4,200 for the year and meet the other requirements under the Qualifying Relative 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 $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.
To claim an exemption from federal income tax withholding on a W-4 -
Exemption from withholding. You may claim exemption from withholding for 2019 if both of the following apply.
• For 2018 you had a right to a refund of all federal income tax withheld because you had no tax liability, and
• For 2019 you expect a refund of all federal income tax withheld because you expect to have no tax liability
