Continuity mistake: At the dinner scene, just before Peter asks his mother if she's okay, the meat on his plate, (apparently chicken) is sitting with the mounted end up. It cuts to his mother then back to him without him moving, and suddenly the meat has flipped upside down the other way. And it flips around several times through this scene. (00:56:55 - 00:58:00)
Continuity mistake: In the classroom when Peter is being possessed, he slams his face down into his desk twice, splattering blood onto it. After he falls back screaming, the camera changes angles and now his desk is clear of blood. (01:38:25)
Answer: As the film progresses, we learn that the mother's side of the family has a history of serious mental illness, and the maternal grandmother was a secret devil-worshipper who was trying to fulfill an evil prophecy (crowning one of The Eight Kings of Hell from her own family). The little girl, Charlie, did apparently suffer some sort of mental illness, and she was raised and heavily influenced for 13 years by her devil-worshipping grandmother. Near the end it is revealed that Charlie actually was, all her life, a temporary female host for one of The Eight Kings of Hell; but she had to die because the prophecy called for a male host to be crowned King. At the very end, Charlie's cursed soul migrates into her older brother Peter's body; whereupon, he is crowned one of The Eight Kings of Hell.
Charles Austin Miller