It could also be based on stories of adoptions gone wrong. There are real cases where families adopted children without fully knowing their background or potential problems. Just like in the movie, the family who adopts the orphan girl faces unexpected and terrifying revelations about her past and her true self.
The real - life events that inspired 'Orphan' are often quite dark. There were cases where children with certain mental and emotional disorders were involved in disturbing incidents. In the movie, Esther's character, who is an orphan, has a complex and dark backstory which might be drawn from real - life instances of children with abnormal behavior that led to dangerous situations.
Hard to say. It depends on the author's inspiration. Maybe there are elements based on real events, but it could also be entirely made up for the sake of the story.