Now I am ready to discus some thoughts I have after reading theories etc. Things no one mentioned:
1. Sebastian-Vincent resemblance. I know the thing that Sebastian has those looks not to frighten Ciel . But can anyone explain me why then Angella and Ditrich don't recognise him?
This means (for me) that Toboso is going to reveal this resemblance (or real identity) as a shock to other characters.
2. Has anyone noticed that Vincent is wearing black gloves all the time. We see his hands only before wedding. After wedding he is ALWAYS wearing them. Even when readin book to Ciel and "Ciel"
3. Phantomhive name. Doesn't it mean Hive of phantoms? I believe all the family members has kind of curse on them. You see here and there that being a Phantomhive means something. Not only being a Watchdog. Before the murders in the mansion we see Vincent and Francis discussing, that she is no longer the part of Phantohive and she is sorry. But why?
Once again, Sebastian shows he is Vincent. Heremembers things only Vincent knows about. I mean photo studio and secret word. Toboso killing me with hints
I still very much doubt that and besides some coincidental similarities there is no real evidence.
The Twin Theory was obvious, but what i read about this "Theory" so far just hangs in the Air.
Or to be more specific, its more than that. While knowing how Yana Toboso writes, she has more in stock by just simply saying Vincent is Sebastion, lol.
It could also be Sebastian serving Vincent as a Demon as well or the whole Situation about Claudia Phantomhive and why Undertaker (kind of) serves the Phantomhives.
Could it be Vincent had a contract with Sebastian before? And the seal was on his hands that is why he always wore gloves?
Maybe, maybe not. Wearing gloves regularly was normal at that Time. It shows nobility and etiquette. Our Boy is wearing them quite often as well.
Uh... that's obviously emergency exits that Ciel and Sebastian made.
If only Sebastian knows about it, that would certainly shock Ciel.
Also.. this manga was planned as a yaoi originally, so I seriously doubt Toboso wanted an incenst story.
But she even pull out an evil twin plot from nowhere so.. what would I know?