Hal_Al
Level 15

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The children may qualify as  dependents, but because they are not related, her children cannot be your qualifying child for the child tax credit, earned income credit or Head of Household filing status.

A person can still be an other dependent (Qualifying relative in IRS parlance, even though they do not have to actually be related), if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with you ALL year

2. His/her gross taxable income for the year must be less than $3,900 (2013)

3. You must have provided more than 1/2 his support

 Whether a qualifying child or qualifying relative:

4. He must be a US citizen or resident of the US, Canada or Mexico

5. He must not file a joint return with his spouse

6. He must not be the qualifying child of another taxpayer

He must have a US social security number or tax identification number (TIN)

In addition to the above requirements, to claim your boy/girlfriend's children, they must meet all of the above requirements and:
--- your boy/girlfriend must not be required to file a return,
--- he/she does not file a return claiming the children

Whether it affects their benefits would have to be answered by the agency providing the benefits.