No. You still need her permission.
The IRS goes by physical custody, not legal
custody. The custodial parent has
first priority on claiming the children on her taxes; regardless of the amount
of support provided by the non-custodial parent. The non-custodial parent can
only claim the child as a dependent if the custodial parent gives permission
(on form 8332) or if it's spelled out in a pre 2009 divorce decree. Even if a divorce decree, dated after 2008, gives the non-custodial parent the
right to claim the child, he must still get form 8332 from the custodial
parent. A properly worded decree should require her to provide that form. https://www.irs.gov/pub/irs-pdf/f8332.pdf
A child can be the “qualifying child” dependent of any close
relative in the household. If she lives with someone else, e.g. her parents, they can claim your child, before you can.
