Such a childish logic. . .is what i want to say. But, because of their parents position, i can't really rule out that they are going to neglect their 'not so special' child because of. . . . .reason.
If you're going to have such a base motive, at least make it more subtle. Don't just dump it like that, it's unrealistic. Her shouting all this exposition mid-fight is just stupid. Naturally drop hints instead of just shoving it down our throats like that. It feels so forced and dumb.
@Luca44
Before finding about the class she was the one with all the attention and afterwards no one would give her the time of the day. That is why she "hates" her sister, she believes the sister took everything from her.
All it took was a single line to remove all the pathos.
The "unwanted child gets killed", her sister comes out with a legendary class so she's afraid she'll get discarded and killed.
Thus she behaves the way she does to avoid becoming the "UNWANTED" child.
THAT would be a nice twist.
But no, the custom was ABOLISHED by her grandfather, then what's the point?
A nice developement leading to a good old-fashioned tearjerking moment ruined by a single balloon.