You cannot file a joint tax return with your fiance. You can only file a joint tax return with someone to whom you are LEGALLY MARRIED.
As for whether claiming them as dependents on your own tax return will affect their eligibility for Medicaid--that is a question for Medicaid.
**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.**
