I feel like it was highschool that changed her sister so hard, like, she probably got shocked with the popular kids (maybe she got bullied like the small girl at Asa's school and decided to change) or the diligent ones and started throwing things out that made her look odd, but her sister didn't have that shock and continued being like always and she hated that, that her sister wasn't the perfect diligent sister that she could boast about with her "pack" .
If you see someone that reminds you of everything you gave up in order to achieve a normal stable life (even at the cost of your dreams like being a singer or an actress) just being herself and trying to pursue her dream without fear, that sting, so (and I know, this is just me rambling) she hated that... Her excuse was that she was protecting Makio of crashing and failing and that it wasn't going to work (society/economy wise) but in truth, maybe she was just jealous because it was working, she is a living proof that it could've worked for her too........... Maybe her sister lived her whole life thinking "I definitely did the right thing".
It might be a big stretch, but I think that part about the bullying was a hint of that.