Humm
Think with me,
You are born, new to this world, knows nothing. Grows up listening wonders and that you will marry your brother. Hearing this over and over again before you could even speak.
She is kinda crazy, no doubt about that, but in her own right per say. I mean, you grow up KNOWING of your future-to-be-husband. Being guaranteed that he is, in fact, your future husband. And all this while this "future husband" is treating you very well (because you are his sister) and very lovingly (Again, you are his sister).
And then, he says he doesn't want to marry you and runs away.
Something snaps.
Betrayal, wouldn't it be?
You are my future husband, so why are you leaving?
Can't say I agree with how she acts, but I'd say that that's how she was groomed to be. If it is to blame someone, blame the parents.
But, well, been a long time since I read this manga (picked up again today). So just saying the conclusion I drew out back then.
I wodering if she was like that from beggins or if it becouse of her bro that she become like that