That girl said all the right things to the wrong person. He is the one who understands all of it, and so he made a move. She rejected. She is preaching to the choir. She should have said all of it to Mizudori, not him. He is not at fault in this situation AT ALL, but somehow it is his job to fix it? Wtf?
Except she said in this very chapter that she doesn't feel like Mizudori would actually hear what she has to say, because of how different their frames of mind are in what they prioritize.
She went to Aoba because she at least knows that Mizudori will listen to
him, because she can see how important Aoba is in her life even beyond the relationship between their siblings.
Aoba reassures her in this chapter that, whatever her own thoughts, Mizudori
does consider her an important person in her life.
But she came to Aoba. Because she felt that Aoba would be better equipped/positioned to get her message to the person who does needs it most.
And to be fair--Aoba needs to hear it, too, to get it fully internalized. He's guilty of the same thing Mizudori is; he just also happened to confess first, but he reiterates just how alike he is in this very chapter, about how he also sacrifices himself and his own happiness for the sake of his siblings.
He's simply slightly ahead of the curve, compared to Mizudori.