**Yana** toboso..
Actually she dropped off hints everywhere. You might want to reread if you missed all of them (granted with the translation there's a few missing though).
Theres some things proven here
http://theofficialdoublecieltheorist.blogspot.jp
by twin hint though I don't mean with the recent volume or arc, since something as huge as a twin shouldnt just be an arc, right? But I'll check.what I missed starting Vol 1.. sleepless nights
It's been there since chapter 2. Starting with ''Ciel Phantomhive'' in quotation marks (it's like that in the japanese version too), Sebastian laughing after he says that.
In Madam red's flashback too, in the original she says ''my nephews'' and in the birth scene she's indeed holding one baby while Rachel has the other at her side.
Then the flashback with the bard in the Circus arc is chockful of it, Our Ciel being all shy is in Vincent's left side, real Ciel and brightly smiling in his right side. And on each panel, either one is out of the frame, or you get a hand and stuff hiding one of Vincent's side.
Also when there's the Easter chapter, when Lizzie remarks at the very end how it's actually the first time she did her own egg. She lied at him, saying he always found it and so he had to this year as well, and so he just went along with it not knowing.
I'm only scratching the surface here.
And actually if you bothered to click on the link you'd see it's not only recent chapters ;)
Three times replying but just reread chap 2, when he says:
"How many times has this ring witnessed the death of its owner? Grandfather, father, and then..."
Referring to his twin. Also, it can be safely assumed he had it when he died, and ourCiel recovered it from his body (he's seen with it being bloodied when he's out with Sebastian, and did not have it previously). It's not translated in the scanlation here but there you go, when Sebastian's Cinematic record is rolling:
http://i.imgur.com/sBpyx2K.jpg
That's kind of more recent but here's that too just to let you see from where I'm coming here ahah:
http://i.imgur.com/EikWNEh.jpg
Dude..!!!! Yes, my bad.. now I remember the nephews part.. OK.. I read the first 5 volumes 7-10 years ago i forgot those little bits.. sharp memory you have.. will check back now.
But that was RIDICULOUS!! Hahaha. Tana toboso, what's up with the sudden twin bump? Like that HUGE twist and no clue, no drop ofnhint, at all it was coming.. hul.. where did you pick that up..