It is revealed that Sarah is Elizabeth's sister but that she more closely resembles their mother. Their mother is actually the demon depicted in the book and on the pagan amulet. The nuns were trying to prevent Elizabeth from realizing her heritage and attempting to free her demon-mother from the walled up crypt she is trapped in. The two women had previously tried to free their demon-mother when they were children but Elizabeth had gotten scared and ran away. She and her father fled the island but it is presumed Sarah has been there ever since, waiting for her to return and complete the ceremony.