There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit and the Child Tax Credit.
A step child (child of a legal spouse) is the same as a biological child and can be a QC. But, the child of a live in girlfriend can only be an other dependent, and even then, only if they lived in the same household all year (other rules must also be met*).
If you get married before year end, she can claim your children as QC dependents. Most likely, in that situation, you would also file as Married Filing Jointly.
*A person can still be a Qualifying relative (Other) dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:
- Closely Related OR live with the taxpayer ALL year
- His/her gross taxable income for the year must be less than $4400 (2022).
- The taxpayer must have provided more than 1/2 his support
In either case:
- He must be a US citizen or resident of the US, Canada or Mexico
- He must not file a joint return with his spouse or be claiming a dependent of his own
- 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 girlfriend's children, they must meet all of the above requirements and:
--- your girlfriend must not be required to file a return,
--- she does not file a return claiming the children
