Anyone can claim a common law marriage. The question is whether one can prove a common law marriage. It has nothing to do with how long people have lived together. Common law marriage is actually a state of mind.
The party asserting a common law marriage must prove the following elements: an actual and mutual agreement between the spouses to be husband and wife; a permanent relationship; an exclusive relationship; cohabitation as man and wife, and; the parties to the marriage must hold themselves out publicly as husband and wife. The party asserting a common law marriage has the burden of proof by clear and convincing evidence.
Each case is fact specific. Filing taxes as a married couple is a strong indicator of common law marriage. Filing taxes as single people who live together is a strong indicator that there is no marriage. That's not the only indicator, though, and not always conclusive. How do the parties handle their other finances? Do they own property together? How do they introduce themselves to their friends and acquaintances? Is there health or medical insurance showing marital status? what about beneficiary designations on life insurance and retirement accounts?
Oklahoma is one of only 8 or 9 states that recognize common law marriage. Evidence of a common law relationship can be contradictory and confusing. You might want to suggest to your mother a cohabitation agreement that states clearly the couple is not married. The agreement can specify who owns what property, and how the couple will share their finances, and what happens should the relationship end.
If we do not have a written attorney-client agreement: I am not your lawyer; you are not my client; this is not legal advice.