There are two possibilities.
You may be an independent contractor. That depends on the relationship between you and the person paying you -- who controls the work hours and conditions, who sets the rules, and so on.
https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-self-employed-o...
If you are legitimately an independent contract, then you file your return as a self-employed person. You will pay income tax plus 15% self-employment tax on your net income. (You pay 15% instead of 7.65% social security and medicare you would pay as an employee.) You must make estimated payments at least 4 times a year or you can be assessed a penalty even if you pay in full when you file your return, so plan on setting aside at least 25% of your pay, possibly more.
If, based on your working conditions, you are an employee, you are entitled to file as an employee and pay less tax (7.65% social security and medicare instead of 15%) but this will blow the whistle on your under the table arrangement. If you want to keep the arrangement, you must continue to file as self-employed even if that is not the case. Your family member is breaking the law and is denying you (possibly) the right to unemployment insurance, the right to workers compensation if you are injured, and possibly other benefits under state and federal law. (It is, of course, possible that you are legitimately an independent contractor and your relative is doing nothing wrong. The depends on the exact nature of your work relationship and duties.)
