Based on everything you have said, it appears that on one can claim the child as a qualifying child dependent on their tax return. Neither you or the brother can claim the child because the child did not live with you more than half the nights since it was born. The mother can't claim the child because she is not required to file a tax return. This means that the child could only be claimed as a qualifying relative dependent which is a different kind of dependent. For you to claim the child as that type of dependent, you would have had to provide more than half of the child's total support since he was born. Support includes housing, utilities, food, clothes, etc. This type of dependent gives you a $4050 dependency exemption but no other child tax benefits. You would still need the social security number to claim the child for this type of dependent.