I know this came out ages ago and I know you have all probably seen it but I hadn't.
This is gloriously, fabulously demented!
Evil child runs amok in seeming perfect but secretly damaged family.
Evil child turns out to be...
(stop reading now if you haven't seen it)
A THIRTY THREE YEAR OLD WOMAN WHO HAS ESCAPED FROM A RUSSIAN MENTAL INSTITUTION!
I kid you not!
What's not to like here?