OK, the test is that she must NOT have furnished more than 1/2 of her support for the calendar year. (This kind of works in reverse than normal in that it is her NOT doing that as opposed to you providing support).
If she was in school for any part of 5 months during 2015, then she qualifies for the earnings exception to the dependency rules.
She must also be under age 24 at the end of the year and have lived with you for over 1/2 of the year. (Temporary absences to attend college qualifies as living at home for that period. She appears to meet that test.)
Also, she must not be married and must be a US Citizen, or resident of Canada or Mexico.
Finally, yes if you were to qualify to claim her as a dependent, she couldn't claim herself and that would increase her tax burden.
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