Ah, okay, that confirms it... Kei is the dark-haired girl in his class. And yeah, I do like her. It's pretty clear neither of those girls he hangs out with normally in school are into him, based on the previous dialogs.
And yeah, I noted last chapter how they were really awkward, and how Tasuku is super forward and over-confident, while Shia is super self-conscious. And yeah, the destroyed script. She's really struggling with her own self-worth, for sure.
I still can't say for sure whether she had been gunning for the older brother, or if they've just been really close like siblings without much baggage- because, again, they grew up together, same as rabidporcupine pointed out above with respect to Tasuku and Shia. The former might help explain her attitude when dealing with Tasuku, but at the same time it could just be the self-consciousness over her self-esteem and perceived lack of ability with her own hobby. Sometimes it really is just as simple as adolescent self-worth issues causing you to not feel worthy of being that close with anyone.
But, still, I feel like the way she responded at the end there was her getting very flustered and running away... again. Just like all those times in chapter 10. The body language has been telling me she's far more self-conscious about how he sees her than the rest of his family, to be honest.