I assume that mother doesn’t file a tax return. If that is the case from what information you posted both she and her child can be claimed as dependents as qualifying relatives (the tax term “relative” doesn’t mean the same as “related to”). But as a qualifying relative the child doesn’t make you eligible for the child tax credit. Only a qualifying child can do that. The first criterion for child tax credit is excerpted from IRS Pub 17 pg. 107.
Qualifying Child for the CTC
A child qualifies you for the CTC if the child meets all of the following conditions.
1. The child is your son, daughter, stepchild, foster child, brother, sister, stepbrother, stepsister, half brother, half sister, or a
descendant of any of them (for example, your grandchild, niece, or nephew).
