supernatural elements
On the surface (until the end) this seems like its not a supernatural movie. The brother and sister (vampires of a sort, though) need blood from victims to keep the brother alive. But they don't bite the victims, they drug them and then drain the blood with a machine to give the brother transfusions. It all seems standard enough until the end when seemingly both Bridget and Jason are killed, but then rise from the dead almost zombie like and continue after Larry and Robin. It does seem to come out of nowhere, but all around the house within the black room there are statues and artworks that seem to indicate magic or witchcraft at play. I think we can take that there is more than just transfusion going on keeping Jason alive, and that some magic is being used, even if not shown. I know one of the statues that Robin sees when she comes to the house looks like a Baphomet. It's probably all just haphazard occult looking stuff thrown together by the set designer, but I don't know if there was any thought involved in what was shown.
Good movie, I enjoyed it.
Short promo trailer for the blu-ray:
https://www.youtube.com/watch?v=gBjE7bBKzZo