Since she is not your child, she cannot be a Qualifying Child for tax purposes where if she were a full time student under the age of 24 her gross income would not be a factor when claiming her as a dependent.
As it is she is not related to you and her age is not a factor but the time lived in your home and her gross income is a factor. For tax year 2019 the gross income must be less than $4,200 plus she must have lived in your home for the whole year, 01/01/2019 thru 12/31/2019.
