“We are brothers who, together, make a greater whole.”
This line is really interesting to me. While there’s evidence to suggest that Sora considers Roxas his brother (why else would Hiro talking about how Tadashi lives on in his heart cause him to see a vision of Roxas hanging out with Hayner, Pence, and Olette), the members of the Heart Hotel have never acknowledged each other as siblings. That is, except for Vanitas.
Xehanort abused Vanitas. There’s no question about that. Ever since Vanitas emerged from Ven, Xehanort treated him like garbage and used him as nothing more than a pawn.
While it’s easy to chalk it up to sloppy writing, Vanitas telling Ven that he’s just something that was sealed away inside of him before referring to himself as a piece of Ven implies to me that Vanitas doesn’t really know what he is. Which makes sense, Xehanort clearly thinks he’s a primordial Darkness but if that’s the case, why would Vanitas be able to feel some of Ven’s emotions? (Also, I think it’s worth pointing out that Vanitas only distances his origins from Ven after he shoots down the thought of them being brothers in favor of them being the “same”.)
Vanitas is a child who was abused for years and who is unsure about even basic aspects of his own identity. But he wants connections and he wants to forge an identity of his own. So he takes a look at the boy with whom his heart is bonded to and at the boy who gave him his face and decides brothers.
Granted, he’s a… very bad brother, to put it mildly. But the fact that, out of all the Heart Hotel members, Vanitas is the first one to address any of the others as his sibling is huge and I don’t think that enough people talk about it.