I was thinking the same too. Last chapter we see the MC encouraged this boy and he got flustered and ran away, so I thought he's definitely gonna be involved in the romance (but not like this). Though in the end the brother will triumph! Or maybe the ending will deviate from the premise? Who knows...She will end up falling in love with the shota and the brother will stay with the friend's sister. I see it coming, and that will be bullshit because onii-chan is the best waifu of this manga.