I wouldn't go as far as to call the MC useless she went through all that harsh training from political education, and learning to be a proper lady and for what? A little sister throwing a temper tantrum because she has to have her way, and a father who can't stand firm to keep his foot down while she's throwing her tantrums. The Royal family are just rotten to the core (sans the second prince who I believe was suppose to married the MC in the first place) came to her defense. So the MC is not useless her family and the kingdom is and that's what leading them to there annihilation. Sorry for the long and late reply.
To think a kingdom comes to ruined because of her worthless father, and her spoiled rotten brat of a little sister.