Pretty much in tune with the general IMDb rating

Rating: 8/10 (29 August 2011)

***mild spoilers***
A close 8 this one is...
It's not the world's most innovative concept, the adopted, evil child manipulates a superficially healthy family by surfacing their past sins and playing them intelligently. The exposition in this movie is a tad too long for my taste but it's bearable since the metamorphosis of the lovely new sister to the evil monster is nicely portrayed. But that's about if for the criticism really.
Apart from all the nay-sayers on the net ranting about how the characters are totally unbelievable (while well played): I assure you; They're not. Most of the appeal of this movie - at least for me - came from the credibility of the main characters. The mother, laden with guilt and unsure about her worth but strongly connected to her youngest daughter and thus quickly realizing something is just "not right" after the first weeks with the new kid. The father, oblivious to the influence the "orphan" has on their kids but unwilling to believe his wife which seems to be having a really bad alcoholism relapse (this part is especially credible). And the children, scared to death by the "orphan" who threatens them with the death of siblings and parents (also extremely authentic knowing how domestically abused children cover their parents' crimes, often for years. In particular how the "orphan" made friends with the youngest daughter to be in a better position to blackmail as much her as her mother.). The final twist is also very well chosen and really gives you the creeps one more time and refreshingly devoid of any supernatural deus ex machina to explain how the girl got so twisted.
So yea, it's the credibility that makes this movie really shine. No demons, almost no paper-cut "hero characters": The psychological coherence is king in this movie, very good... (This comes BTW from someone with experience on clinical psychiatry)
I want to emphasize again though: Do not under any circumstances let your children watch this movie. I mean it!