the twins’ blood type is Sirius and Sirius is a binary star which is literally called “twins star” (双子星) in Japanese. (Afaik Polaris, Vega and Canopus are not binary stars).
the twins’ father’s zodiac sign (i.e. Vincent’s zodiac sign) is Gemini which is called “zodiac sign
twins” (ふたご座) in Japanese.
and lastly, the 2CT was revealed on June 17 which is the month of Gemini. Again, in Japanese it’s more obvious because Gemini is literally the “
So even though the word “twin” (双子) was never mentioned in the story, Ciel’s profile (his birthday, blood type, etc) has been heavily hinting at the existence of a twin all this time. And I guess that’s the very reason why the Japanese fandom (me included) is so convinced that the real Ciel we saw in the latest chapter /must/ be our Ciel’s twin brother and therefore doesn’t even consider the possibility of a sibling, imposter or a clone at this point! xD