i would not really say that. twins are really close. some of them even think that the other is their other half or something. i think ciel just loved his brother too much and maybe even felt guilty of the fact that he was the only one sick. so he tries to spend as much time as possible with him to make him feel better. honestly, if you had a twin that tells you that he wants to leave when you guys grow up, will you agree? i think i will even be willing to marry the same man if i had one( ̄∇ ̄")
There already a theory about the real ciel being the one to plan the death of the parents to prevent our ciel from leaving him, and he could've gotten help from undertaker who already knew in the beginning, hinted in the Sullivan arc (or the cruise one I forgot the name) that our ciel is not the one he knows, Even saying that the "real one will appear" something in the lines of that.
Oh gosh darnit they're soooo adorable aaahhh! ε=ε=(ノ≧∇≦)ノ But the thing is that, the real Ciel feels a bit too ominous, even as a chid. Is it just me??