Just because our MC is in middle school, we shouldn't forget that Shia herself is only in high school as well. I think she feels guilty for hanging out with him when she likes his older brother, and is therefore doing this self loathing act whenever he mentions how he likes her
I dunno... she has seemed self-conscious in a way that seems understandable around someone you're interested in with Tasuku (MC). The way I see her with the older brother, Tsumugu, kind of feels like she actually does think of him as her own brother so doesn't really think much of showing vulnerable sides, but for whatever reason the little brother is different to her, to the point she gets flustered showing other sides to him.
The chapter where she kept bolting off around Tasuku really felt like it hammered it home, to me. So flustered she couldn't stand to be near him for all that long until she finally had to confront him.
And in the case of the screenplay, to me, she seemed less concerned about if Tsumugu gives her harsh criticism, but is absolutely terrified of letting Tasuku see the same and suddenly disliking her and never hanging out again. Yes, she tries to dissuade him, but I feel like it's just adolesecent self-esteem issues, especially over her hobbies, and worrying about him not liking what he finds if he digs too much into who she really is.
You could be right, but I do think there's room for my perception, as well. Of course, you probably know more certainly at this point, having probably finished this and I'm only just now at this chapter.
Either way, I'm keeping an open mind moving forward reading, but trying to keep positive. If she really does like the older brother like that, I figure it is not reciprocated.
Especially with the way the older brother seems to have been so far in relation to his younger brother and Shia, I almost want to say he's actually supportive of them getting together. In fact, both siblings seem to have been, very subtly, helping, in their own ways. Third chapter was an especially important one for seeing how much Tsumugu surreptitiously does for Tasuku, if you pay attention. And last chapter before this one felt entirely caused by his sister, Shizuku, inviting him to the Starbucks because Shia was there, but without outright stating that's why she wanted him to come. There was literally no reason for her to call him there, unless it was so he could stumble across Shia. It's less obvious if she knew those other two girls were going to cause trouble, but it is possible, ergo there is a remote possibility he may have been brought there to come to Shia's rescue by his own sister.