my personal feelings on dad nier is it contextualises the violence of the game better in terms of a commentary on the possessive violence of the father, i just don't think that stuff hits as strong with a brother as the cultural dynamics are immediately so different. and also it completely changes the dynamic between nier and weiss. brother nier is young and naive and loses that in the timeskip, whereas dad nier isn't. he's kind out of a kind of exhausted pragmatism, and so i think his dynamic with weiss is sublty so much better because his arguments against weiss being a fuckin asshole come from his own experience and not youth.

but yeah basically those are the short version of my two reasons. i'm sure brother nier still works but the context is so radically different that i wish they'd been able to keep both