You cannot claim them as "Qualified Child" dependents for the EIC or Child Tax Credit because they are not *your* children and you are not married.
You can only claim them under the "Qualifying Relative" (or other non-related dependent) of you meet the requirements. That will give the the $500 other dependent credit and nothing else.
In TurboTax you must enter them as "Other person", NOT your child.
The "proof" they want is a copy of the child's birth certificate with your name on it and/or a copy of your marriage certificate to show that you are the step father.
As your fiance's children are not your biological children, they are claimed as Qualified relatives. They are not eligible for the Child tax credit or the Earned Income credit.
You can claim an unrelated person as a dependent (qualified relative) if he/she passes all of the following tests:
- The person can't be your qualifying child or the qualifying child of any other taxpayer.
- The person must live with you all year as a member of your household (and your relationship must not violate local laws).
- The person's gross income for the year 2020 must be less than $4,300.
- You must provide more than half of the person's total support for the year.
- The person is a U.S. citizen, U.S. resident alien, U.S. national.
- The person is not a married person who files a joint return unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid.
The IRS is asking you to prove that they satisfy all these conditions, mainly that they are US citizens, residents or nationals, that they lived with you all year, and that you provided more than half of their support.
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