AN: based/inspired by several things that keep popping up in the manga that have not been explained in the chapters available to me.
Disclaimer: I do not own black butler and make no claims to know what the wonderful Yana Toboso is planning to do with the hints given. This is just a guess to keep me (and others) entertained until she reveals all (which hopefully she will do, soon, for the sake of my sanity.)
Every child has an imaginary friend and Ciel was no different. Despite being surrounded by people whom he loved he still followed that child's instinct to create a person who only he could see and who he could always rely on. When he announced to his parents that he had an older twin brother who was identical to him his parents did what any other parent would have done, went along with it while privately dismissing the notion. All children create and depend on a person who no one else can see.
The difference between Ciel's 'twin' and other children's imaginary friends was simple. Anyone could have seen Ciel's twin if they had decided to look closer.
The funny thing about humans is that if they do not believe in a possibility they will dismiss anything that has the potential to prove it. They will not notice that the small child following his parents has a slightly darker shadow than those around him. People do not notice that sometimes when they are talking to this breathtaking boy he is instead a mirror image to the young master.
Every child has an imaginary friend but Ciel's was different, Ciel's was not an illusion but an actual demon.