ha-ha, this is, like, essence of almost all school romcoms!
I get the impression that big bro understood that she's autistic, and that's why he talked with her in terms of "compete" and "win". And just like that, he successfully planted the idea of marriage into her brain. Well done, big bro!
And yeah, I understand Shun's position about mikan. I know this very well. If there's something lying around the house, I'd be eating and eating it. Even if it's not something I relly like. And just like that, it'll be gone
But man...her face at p.10. She was probably thinking that she'd never see him smile like that.