If I understand your situation correctly, your parents are divorced and the divorce court gave your mom sole custody but for unstated reasons first you wound up with your aunt and uncle and now with your dad, and want to go back to your aunt (and uncle?). The divorce court would have to be involved. You are 16. Illinois law may allow you to act relatively independently but you'd have to go to court. If your aunt and uncle are the persons willing to help out and take you in, that's potentially a big help. But you will need a lawyer who deals with custody cases. Sorry to hear about that apparent lack of stability in your life. Good luck.