I am unaware of the laws governing under aged wages and the associated parental rights. I would assume your only real chance at finding answers is to approach a lawyer or someone else qualified to answer your questions.

Don't give up on your talent because of this. Use your frustrations to empower your artwork and become more in demand. When you gain control over your own funds you can tell your mother that you have paid your way thus far and now it's her turn to pay for the privilege of having you in her life. Best of luck.