1) It wasn't the same house I don't think, just a neighbouring apartment.
2) They were there to be consumed by "The Beast", the same as Casey
3) The "reveal" was that these events took place in the same world as *Unbreakable*. M. Night Shyamalan plans to make another film which has Bruce Willis' character from *Unbreakable* face off with "The Beast".
4) That would have been a good twist. I assumed that the twist was going to be that Casey also suffered from DID, and that the other two girls were just alt personalities of hers.