(a) She's a dude.
(b) She was dead the whole time.
(c) She's Macaulay Culkin.
Having seen Orphan, I can state with complete confidence that Esther's secret is one of the above or something else entirely.
You know what else I can state with complete confidence? That Katherine Heigl looks like Mr. Bill and hates women; that the computer-animated guinea pig you think is voiced by Flavor Flav is actually voiced by Tracy Morgan, which makes you a racist; and that Orphan (at the time of this posting) has a higher positive-rating total on Rotten Tomatoes than do The Ugly Truth and G-Force combined.
Also, anyone over the age of 17 who voluntarily goes to see Harry Potter and the Half-Blood Prince is a pedophile.