No, you cannot just keep her in the US or petition for her green card as you are not her parent and as an aunt not a qualified family relative for immigration purposes. If you adopted her before she was 16, then you could have petitioned for her as your child. Otherwise, she can try to get a student visa as a high school student and then continue with college as an international student.
Answered on Feb 05th, 2014 at 2:55 PM