I'm so sorry she's in such pain. And she is in emotional pain; she wants mommy and mommy doesn't seem to want her, in this little girl's mind.
If she is on Prozac, she is seeing a therapist -- isn't she? I hope she is not just taking the medication; it should go hand in hand with very regular visits to talk to a therapist who specializes in children who have been separated from their parents and children who have anger control issues. If she has a therapist, what you say here should be said to that therapist -- whom you should be seeing too. It's very important for her therapist to know what goes on at home between office appointments! The food stealing, yelling, refusal to do anything -- those all scream out for professional help, both for her and for you. You should be seeing someone who can give you advice on what to say to her and what to do (especially because sending her to her room clearly does NOT work--that's sending her to her favorite place with all her personal stuff right there).
You say she and her brother were "placed in my home" so I assume this is some kind of foster placement? Are you a relative? Do you have some form of legal custody or guardianship? If they were placed with you and it's fostering, the agency that is in charge of the foster care arrangements should be able to direct you to counseling for you and for her.
If she is not seeing a therapist for regular sessions, please, please get her to someone. If you can't afford it, talk to that fostering agency, talk to your county or city mental health department, talk the counselor at her school. Ask about clinics and therapists with low-cost or sliding scale fees. The defiance and anger really do need more help than most parents or foster parents can give day to day when you have to deal with her acting out.
You are doing a really good thing by taking in her and her brother, whatever the circumstances are. You can help her through this (though you cannot make her mom act like a mom, or make her mom be consistent in visits, sadly). But do reach out for all the professional help you can. Start with the school counselor if you aren't sure where else to start. If there's already a therapist involved, talk with that therapist yourself and be sure he or she knows what you're telling us here, and gives you some advice.