User blog comment:ImperiexSeed/Castiel's shifting of Sam's mental problems made no sense!/@comment-4920092-20120402024406/@comment-3481504-20120402025737

Kind of. Well, Cas bears him (Lucifer) now, so the hallucination just switched, resulting in him calling Cas brother.

Though it is kind of weird how Lucifer acknowledged Cas. But when it comes right down to it, he is just a figment of their imagination, he's still in the Cage.