Responses
I don't believe the twin theory - I think it is over analyzing and the anime getting away from the manga (as often happens). There's no real evidence of a twin (more of a conspiracy theory for now), and there's no reason whatsoever for Ciel to pretend to be his twin. Also, the family would have said something - they only ever mention his parents to him/others...
I'd always thought that the twin Ciel theory was a bit pretentious and that people were just over-analyzing things like with FNAF...but now... Hmmm I'm starting to sway on the idea. I still think undertaker is grandpa phantomhive, but that's my own pet theory.