My 12 year old niece' whole personality has change in the last few weeks and she has been saying that it is the voice of the devil, can u help me?

See a doctor: She should see a child therapist as soon as possible. She should also be evaluated by her primary care doctor to see if there is any medical situation that could explain this- such as poisoning, toxins, or an unusual infectious process.

Sounds ominus: If she is hearing voices (devil or others), she needs evaluation by mental health professional. There are many reasons for children to change personality, but there may be serious problems, social or medical. If possible, make sure she is seen by someone skilled in child mental health disorders. This may be the tip of the iceberg.