Physical custody is key in determining who is eligible to claim a child. The fact that her Mother has court issued custody does not create the eligibility on her tax return to claim the child as a custodial parent.
A step parent has very similar rights like the regular parent. Included here are the Qualifying Child Rules for your review to make a determination. There are tie-breaker rules that the IRS will use if more than one person claims the same child. Be prepared with proof documents such as the child's address on school records, doctor records, anything that shows proof of address and contact person in emergencies and/or guardian.
