Who did Sebastian the dog really belong to in the Phantomhive household? One twin? Both twins? or family dog? And which twin was frightened of marrying someone that was scary as Aunt Francis, in the flashback scene? And when Lizzy wondered what has happened to Ciel in that one month, the manga used the word "You" instead of the words "Both of you". Why was it that if Yana did plan for Ciel to be twins all along, that no characters ever asked him, where his brother went?
I think the dog belonged to Vincent. But it was a family dog so it basically belonged to the whole family.
The one in Lizzy’s flashback scene was the real Ciel.
And when Lizzy wondered what happened to ‘Ciel’ in the Easter chapter…
…she was thinking about the Ciel who returned (our Ciel who she believed was the real Ciel) since he is the one she worried about.
As for the reasons why no one ever mentioned a twin I guess one reason was for plot convenience. But in Japanese, it’s not always so clear if one is talking about one person or two. So sometimes when someone talked about the ‘child’ it may have also been the plural ‘children’ and in such cases Yana intentionally didn’t specify it so that it could have meant both. I don’t speak Japanese myself but @akumadeenglish has written many interesting posts about that (see here for example). Also, there were instances when indeed the plural form was used.
Another reason may be that no one wanted to make our Ciel remember that horrible month. Sure, they mentioned his parents and that was one of the points I found a bit weird, too, that no one mentioned the twin. However, we don’t know what they all talked about during the time before the manga started (which was almost 3 years after what happened). Also, if most of those who knew both twins were actually aware of our Ciel’s true identity (Tanaka, Frances, Madam Red,…) maybe they didn’t even mention the twin since they thought our Ciel had his reasons for taking over his twin’s identity and didn’t want to startle him by mentioning the twin.