@MikeFD The child of a "significant other", GF or BF's child who lives with you cannot ever be a qualifying dependent that enables you to file as Head of Household or to get the child related credits like earned income credit, child tax credit or stimulus payments. All you "get" for claiming a child who is unrelated to you who lived with you for the whole year is the $500 credit for other dependents.
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
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