@Countrygirls - Her income is not relevant.
There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit and the Child Tax Credit (if under 17).
You say you pay child support, which indicates she doesn't live with you. But, since the divorce decree allows you do claim her, you can still do so*. Her income does not change that. But, this also assumes that she did not pay more than half her own support during the year. Money she puts into savings does not count as support. Note that the requirement is not that you (and your spouse) provided more than half her support (as required for a standard dependent) but only that she did not provide her own support.
*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 IRS goes by physical custody, not legal custody. 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. (without conditions - usually the payment of child support). Even if a divorce decree, dated after 2008, gives the non-custodial parent the right to claim the child, he (they) 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
