She is over age 18, not a student and makes more than $4150; so she cannot be your dependent.
The more than 50% support rule ("I pay more that half her expenses"), alone, does not qualify her.
There are two types of dependents, "Qualifying Children"(QC) and
standard ("Qualifying Relative" in IRS parlance even though they
don't have to actually be related). There is no income limit for a QC but there
is an age limit, a relationship test and a residence
test. Only a QC qualifies a taxpayer for the Earned Income
Credit and the Child Tax Credit.
A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:
1. He is under age 19, or under 24
if a full time student for at least 5 months of the year, or is totally & permanently
disabled
2. He did not provide more than 1/2 his own support.
3. He lived with the parent
(including temporary absences such as away at school) for more than half
the year
A person can still be a Qualifying
relative dependent, if not a Qualifying Child, if he meets the 6 tests for
claiming a dependent:
1. Closely Related OR live
with the taxpayer ALL year
2. His/her gross taxable income for the year must be less than $4150 ($4,050 in 2016-17)
3. The taxpayer must have
provided more than 1/2 his support
In either case:
4. He must be a US citizen or resident of the US, Canada
or Mexico
5. He must not file a joint
return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying
child of another taxpayer
