What's the general rule with driving a relatives car?

In the United States, I'm driving my moms car since I'm having mine worked on soon, it's recommended that I don't drive it unless i have to. Basically the mechanic said not to drive my car unless I have to, so been driving mom's car for the time being. Mine goes in 6-1-22

We both have the same last name and she has insurance and the car is registered. She just hasn't been going out as much.