If you and your GF are not legally married, you can only claim her children as "other dependents" for the $500 credit for other dependents----IF they lived with you for the WHOLE year and if you paid for over half of their support. You do not mention if GF has income and would be claiming the children so we cannot say if you would even be able to claim them for the $500 credit if you are living together.
Unless you are their legal step-parent, you cannot get earned income credit or child tax credit by claiming your GF's children.
If you and GF become legally married, step-children can be claimed as dependents on a joint tax return that you and your spouse file together. If you are legally married, you (and your spouse) might be eligible for earned income credit and the child tax credit and additional child tax credit. You are considered to be married for the entire year by the IRS even if you get married on December 31.
However---we still do not have any information about whether there is another parent who might also be able to claim the children---more information is needed.
**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.**
