i did have the same feeling a bit that other characters (like lizzy, madam red, etc) hardly gave any indications that ciel has a twin.

there were of course hints from sebastian & ciel themselves, but no one else gave any major hints. i thought it could be explained away that they didnt want to upset ciel as he mostly gave bad reactions to when people mentioned his family

the one time i thought id see a hint tho was with madam red thinking about ciel's birth before her death. she said she helped deliver him, but there wasnt any mention of the twins. (if i remember correctly) there were frames that couldve hid a twin to just show one, but why not mention them at least?

i did reread the series shortly before the twins were revealed because the theory was going around, but the madam red scenes pushed me away from the theory for a bit as i thought it couldnt be explained away as the rest of the family's interactions could have been