Possible Spoiler Warning
Here's a good rule to pick movies by: If the TRAILER is already talking about a Shocking! Twist!! Ending!!!, it's probably not very good. If they thought it was good enough to stand on it's own, they wouldn't feel the need to imply that you might be missing "this year's Sixth Sense."
In full-disclosure: I went into "Orphan" 99% sure I knew what the "twist" was, having heard of a certain producer making a film with a certain "really dumb" final shocker a year or so ago. Thusly, I can't tell you whether the "secret" is as easy to figure out as it is dopey; other than to opine that it's one of those things that you'll only guess if you've heard of "it" before. In this case, I think that especially devoted fans of either "Law & Order: SVU" and/or "Batman: The Animated Series" will probably call it right off the bat. Make of that what you will.
Whatever. The fact is, knowing or guessing the "twist" doesn't really effect the film much - this isn't like "Psycho" where the big reveal explains a central mystery and redraws everything you thought had happened. Here, it's just a lurid "oh by the way" that mainly serves to take the "teeth" out of an otherwise potentially-disturbing sequence it's intercut with toward the end. Otherwise, this is just a 100% by-the-numbers, unimaginative "spooky kid" movie. Title baddie is overly well-mannered and dresses like an American Girl doll? Check. Marriage recovering from recent tension? Check. Kid with exploitable disability? Check. Knick-knack of irreplaceable sentimental value that might as well be labeled "break for purpose of of escalating tension? One parent has job that keeps them out of touch constantly? Check. Ridiculously-ornate house in the middle of fucking nowhere? Check. "Audience POV character" with dodgy "nobody trusts me" past? Double-check. There's not a single thing that happens here that you won't see coming.
"Spooky Kid" movies, of course, belong to the subgenre of exploitation movies I've occasionally liked to call "projection monster" films - wherein the "monster" is a generalized version of a certain type of person or identity that most of us never get to "confront" the way we secretly want to; usually for purposes of social correctness (see: "Evil dad" movies, "evil cop" movies, etc.) The ultimate expression of this is the Zombie Movie, wherein the form of the monster allows for the good guys to lay indiscriminate waste to humanity itself. That's why "spooky kids" are always prim and smug, as though they're fully aware of how much you HATE bratty kids sometimes. Don't lie. You know exactly what I'm talking about. Ill-trained little rugrats running around causing chaos, screaming infants inexplicably brought into movie theaters... there are times you just fuckin' HATE children, and "spooky kid" movies are an outlet for that - particularly seeing them take the innevitable beatdown at the end (in fact, one of the few marks in "Orphan's" favor is that it has seemingly no qualms about inflicting physical harm on it's population of obnoxious tots.)
In any case: Vera Farmiga and Peter Sarsgaard are the mom and dad. She's a recovering-alcoholic, he's an architect, she got drunk and nearly allowed one of their kids to drown, he's straining under the weight of being the "stable one," they're both getting over a recent miscarriage. Their son is an angry little shit, their daughter is a hearing-impaired cherub. The titular adoptee is Esther (Isabelle Fuhrman) a Russian refugee who's foggy past has evidently imbued her with advanced skills in the fields of piano-playing, painting, period dress and never being seen by doctors (got it yet?) Bodies, injuries and coy manipulations are piling up before you know it, and poor ex-alkie-mom can't get anyone to believe her.
At the very least, it goes at the material feet first: The (very) R-rated gore helps give the third act a bit more heft than it deserves (since you can't be 100% positive people won't be killed) and the actors are all really good. A mountain of buzz will fall on Fuhrman, who's good but not exactly a revelation - how many times can you see the "I'm too glib and well-spoken for my age and therefore you should be unnerved" schtick and still be impressed? Aryana Engineer, a newcomer who plays the deaf, sign-language speaking younger daughter (she's apparently partially deaf in reality) has the harder part and shows real range... though I hope she was unaware of the context of most of her scenes. The "twist," sadly, ends up sucking a lot of the edge out of things... it almost feels tacked-on in order to make Esther's "bigger plan" less creepy to the audience and "justify" the eventual punishment she takes.
It's not very good, is the bottom line; though I imagine it'll be pretty impressive to younger audiences who never saw Bad Seed or Omen.