She can be your dependent if:
(1) She earned less than $4,050 in 2017.
(2) You provided more than half her support in 2017.
She is considered a qualifying relative in this case, as opposed to a qualifying child, which has different rules, one of which is the age rule to which you are referring.
See the following worksheet to determine if you provided more than half her support. In particular, her reliance on the savings account may disqualify you.
https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
She would need to be a "Qualifying Relative." Relative does not have to mean blood relative.
Under the qualifying relative rules, a person can be a dependent if:
1. Not qualify as somebody else’s qualifying child or qualifying relative
2. Live with you the entire year (365 days) or be one of these:
Your child, stepchild, foster child, or a descendant of any of them
Your brother, sister, half brother, half sister, stepbrother, or stepsister or a descendant of any of them
Your father, mother, grandparent, or stepparent, but not a foster parent
Your son-in-law, daughter-in-law, father-in-law, mother-in-law, brother-in-law, or sister-in-law
Your uncle, aunt, nephew, or niece
3. Earn less than $4,050
4. Receive more than half of his or her support for the year from you
To claim a dependent, these must also be true:
He or she can only have filed jointly with his or her spouse to claim a refund of the taxes withheld. Also, if he or she were to have filed separately from his or her spouse, neither would have owed taxes.
The dependent is one of these:
U.S. citizen
U.S. resident alien
U.S. national
Resident of Canada or Mexico
You can’t qualify as a dependent on someone else’s return. This is true even if the other person doesn’t claim you on his or her return.
