I think he impersonated him because as his father said the younger brother won't ever be the earl, and so to be able to inherit the title as the earl and get his "revenge" he needed to be his brother.
Yeah that is one of my thoughts on it too, they seemed so close before though. I wonder what happened to the real Ciel after the fire. So many questions need answering.
real Ciel was sacrificed and fake Ciel used his grief to summon Sebby and plot his revenge. and he could do those things by being CIEL because everyone knows he was stronger.
I'm a bit shell shocked with these past couple of chapters. I can't help wondering why our Ciel impersonated his twin. Could it be that he was sickly and doing the contract with Sebastian made him strong again? Maybe he didn't want his kind brother to suffer at the hands of that cult group that tortured him? Or possibly, he wanted to be in control and be the "older" brother to reap the benefits? He was also able to come out of his shell and be a stronger presence than he was when he was sickly and younger.