dude did you even notice how he look at her from the very start? it's not how you look at a sister XD and they didn't grow up together so I don't see what the problem with Yus being the ML. he is cool, handsome, very caring for her has his lover to be and not has a sister.
and the noble's society never seen them has bro-sis anyway but has enemies since they not related by blood. I really don't see what your problem is with this story.
I would probably okay with it if she stopped calling him brother all the time and actually started to have feelings for him idk. I like Yus in general it just bothers me that she thinks of him as a brother while he’s trying to be more or probably isn’t even aware of his feelings yet. Also do the nobles see them as enemies cause she introduces him as brother and calls him that pretty frequently. This is just my opinion of course.
to be fair, she comes from another world after reading this plot. she just acting according to what she thinks the plot is, she doesn't really see him as a brother since again he isn't her "real" bro from her world. so to her, he is mostly a stranger to her that is very handsome ╮( ̄▽ ̄)╭ I doubt she sees him as a bro if she blushes every time he touches her.
Is Yus the ml? I really hope not I don’t really like those type of incest but not incest stories it would suit them better or be siblings