Your GF is not a "step-mother" since you are not married. Since your roommate is not biologically or legally related to your children, the only child-related credit she may be eligible to get by claiming them as dependents is the $500 credit for other dependents. She cannot use them as qualifying children to file as Head of Household, get earned income credit, child tax credit or dependent care credit. Your tax preparer was correct about that.
CREDIT FOR OTHER DEPENDENTS
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
If you lived together for the entire tax year, your roommate may also be able to claim YOU as a dependent for the $500 credit, if you did not have income exceeding $4400 in 2022--not counting any Social Security. That $500 credit is not a refundable credit; it lowers the amount of tax owed.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
