Can u claim a child that is not legally your stepson but has lived with you the whole year?

[xmasbaby0]

If the child lived with you the entire year then you can only claim the child for the $500 credit for other dependents----no other child related credits.   No child tax credit, no earned income credit, no childcare credit, no EIP payments,  no Head of Household filing status.  Only the $500 credit, since the child is not related to you.   If you were legally married to the other parent, all of that would change and you could file a joint return and get the child-related credits.

Even if the child's mother did not work in 2021----SHE can get the child tax credit and the 3rd EIP payment for the child  IF she is not claimed as someone else's dependent.

[LenaH]

It depends. You may be able to claim the child under the qualifying relative rules.

To claim the child, all of the following conditions must  be met: 

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year.
• They made less than $4,300 in 2020.
• You provided more than half of their financial support.

For more information, please see Who can I claim as my dependent?