With respect in trying to explore gender identities to Fujimoto, I find this writing to be distasteful because of what the brother says, which seems to be the morale of the story.
"If you are a girl then stay put, but if you are a man, go after her"
This line simply reinforces gender stereotypes and god, "If you're a man, you must never wash your asshole cuz it's gay" or "If you're a woman, you must always do the dishes", I don't think we need more of those. This manga even tries to poke fun at the stereotypes by showing that, even before being transformed into a girl, the boy still had "feminine" traits. So idk what this manga is supposed to even convey.
I would've preferred if he instead said something like "It doesn't matter if you're a boy or girl, but if you truly care for her, go after her".