Take her to her GP and get a referral to a psychologist specialising in eating disorders in children. Unfortunately it needs to be caught as soon as possible for treatment to be its most effective and your niece is definitely showing the major signs of anorexia. I don't think it is a "phase" as she is obviously exerting considerable will power not to eat.

There is nothing that you can do that will be effective without specialised help.

Please, get her the help she needs as soon as possible.