Depends on state law. See this info https://secure.ssa.gov/apps10/poms.nsf/lnx/0200305075
a joint return does not establish a common-law marriage. A minority of states recognize common-law marriages. But, even in these states, it’s not enough to just simply live together for a certain amount of time. Instead, the couple must hold themselves out to the public as married persons. There are various ways that can be done. Each state that recognizes common-law marriage sets forth certain tests that must be followed to establish the relationship. see a lawyer or a financial advisor because of her Social Security benefits. if you meet the state's requirements for recognition then a joint return or married filing separate return should have been filed for each year the marriage existed. there is no way for us to know how a joint return would affect her Social Security benefits. you don't have a choice either you have a CLM or you don't.
Common law marriages are recognized for federal income tax purposes if they are recognized by the state in which the taxpayers reside. If the taxpayers later move to a state which does not recognize common law marriages, they are still considered married for federal income tax purposes.
Most states do not recognize common law marriages. If you live in a state where that is the case, you are not permitted to file as a married couple. I am not sure what is the applicable law if you do live in a state which does recognize a common law marriage. Do a google search for your state to see if it does or does not recognize common law marriages. The children would not be affected for purposes of SS survivor checks but your girl friend would most definitely be affected if you are married. Survivor checks would end for her - but not the children - if you two got married or if you declare yourselves to be common law married in a state where that status is permitted. It sounds like you would lose more money than you would gain. She probably qualifies for head of household status.