Unfortunately, the only way you could get a tax benefit for this situation is if you can claim her as a dependent.
However, if there are multiple siblings that are helping with her support, and the combined support is greater than 50% of her financial support, you and the other siblings that provide support could enter into a multiple support agreement.
In a multiple support agreement, a group of taxpayers (usually a group of siblings for an aging parent) provide more than half of the support (but no individual provides more than half alone) to an individual. The members of the group sign a statement that allows one member of the group to claim the exemption each year.
