@Nayn
The son has been studying ever since he was a little brat, but the daughter only needs to start to take life seriously when she's 14. In fact if she had shown interest in studying at any point, I believe those happy parents would have allowed her to study and assumed she takes it seriously. But obviously she was only interested in martial arts and playing around.
@Kaarme Agreed. They're demanding of him as the first son, but the daughter has actually basically been doing whatever she wants for quite a bit longer.
Being marriageable is hardly a trait that stopped being desirable after the medieval period. (In fact, it's only more recently that people *coughfeministscough* have started trying to undo the practice, and I think many wouldn't agree it's been wholly beneficial)
Just want to say that the condition of the sister here is still way better than the condition of girls in certain country right now in the 21st century. So no it's not only the medieval societies. Or I guess you could say that some societies stayed in the Middle Ages.