My Sister Is 12, Almost 13 And She Does Things

My sister is 12, almost 13 and she does things that 3 year old's do. She puts things in her mouth, stomps her feet, doesn't listen to anyone, screams when she doesn't get her way, won't go downstairs in the house unless someone goes with her and the lights are on, has really bad personal hygiene, and steals all the food in the house. What could this be? Please, I am desperate to know and no doctor will diagnose her.