I think it's because Ciel knows that's not actually his real brother, there has been theories that he is not Ciel's brother but a fake puppet the Undertaker made. Ciel even thinks that as well, from what I've read.
He has probably been lied to about it by the undertaker who revived og Ciel in the first place
No, I don't think he thinks that and Real Ciel has his own trauma and deals with it in his own way and he was always a condescending brat but his trauma made his bad traits worse.
I don't think Undertaker lied to him in any way, he probably was very upfront about everything that happened because he was very upfront to Doll about what happened to her family. Real Ciel knows that the satanists sacrificed him and Undertaker probably told him what happened after he died and that's why he hates Sebastian so much.
It is him though? Real Ciel is a advanced bizarre doll and Undertaker said he took his body out of the fire on that day.
So I'm not really an action/supernatural/fantasy manga reader but I'm always reading spoilers of my fav anime (lol). I read the part about Ciel's childhood with his twin bro and I'm wondering, what happened between them to make their relationship so sour? Like shouldn't the Ciel we know be happy that his bro is still alive? They were so protective of each other and loved each other so much and then BAM, older twin Ciel just set him up is it because he thinks younger twin Ciel really sacrificed him??