She must have less than $4050 of taxable income for the year and you must provide more than half her total support. Here are the complete requirements:

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  taxable income for the year must be less than $4,050 in 2016.

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, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.