Question: When Esther is being taken to her new home, Sister Abigail is seen with her fingers crossed behind her back. Did she do this because she knew how evil Esther was and she wanted to keep that fact a secret, or because she knew Esther's secret and was actually lying about Esther in hopes of getting rid of her more quickly?
Answer: I'll admit, likely due to my own personal experience with adoption agencies, I assumed the fingers crossed behind her back was to hope that it turns out OK in general. Adoptive parents sometimes undo the adoptions for various reasons and give the kids back. Given how many kids they take care of at that orphanage and given that kids in group homes like that are often considered non-adoptable (as they're often put there as a last resort when nobody wants them), I imagine she's probably had several kids returned in the past. It's especially more common with kids from other countries, which Ester is. Given the sinister nature of the movie, I could be completely wrong, of course. I'd love to hear the director's answer to this, as that's who probably would know the real answer.
Answer: I would say your first guess is correct, Sister Abigail suspected Esther's true, sinister nature while everyone else saw her as an innocent child. Later in the film, she tells the mother her suspicions but was never able to prove Esther's wrongdoing. She may have also been crossing her fingers hoping that it was the last time she would ever have to see Esther. It's highly unlikely Sister Abigail knew Esther's secret or else she would have alerted the authorities.