Not entirely Rudeus' fault. If anything the only other stake Rudeus has other than being family is being accomplished. If he's oblivious, we also need to bear in mind that they've been separated because of the teleportation incident for a long time. He wasn't even there when his sisters were born because he was sent away. He wasn't there to watch them grow up in any capacity before the incident, so he never got the chance to know any of them until now. (We could even say that this blunder was caused by Paul again.)
Norn herself was always compared to Aisha, which is what started that ball of insecurity rolling. This is how comparisons can become a bad thing, when using others as a measuring stick of what another person should be able to do by other people's reckoning. At some point, it begins to feel like everyone is sneering at any of the attempts you'd make at all. Because of how Aisha speaks to Norn, Norn may even see it as her half-sister is looking down on her for being inferior to her. Making her afraid of the scorn, and not willing to communicate because she's expecting scorn is all she'll get. Being compared to her older brother just makes matters worse. His own hardships don't mean anything to her because she feels isolated and alone as it is. Her feelings are what is currently right in her face, and her own emotions have her in a choke-hold.
The blame I assign to the students and their homeroom teacher is being thoughtless about making those comparisons, and not considering how many others around Norn were making similar comments, or how even a single one affects her. They only saw their own part, and until Rudeus called out to the class, probably didn't realize they were all doing it.
To Norn, the world has turned against her for not being as capable as her siblings, and that her own flesh and blood hate her for it.