The IRS goes by physical custody, not legal custody. So, to the IRS, her Mom is the custodial parent and has the primary right to claim her. But her Mom can relinquish that right to you by providing you with a Form 8332, which you must submit with your tax return. Here is a link to that form: https://www.irs.gov/pub/irs-pdf/f8332.pdf
There's more info here: https://turbotax.intuit.com/tax-tips/family/what-is-form-8332-release-revocation-of-release-of-claim...
And here: https://ttlc.intuit.com/community/accessing/help/where-is-form-8332/00/26047
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
