Gurl, you undermine you brother in front of everyone, and you want us to believe he is capable of being Duke if he can't handle his overbearing sister. Hell, the page and knight talk down to him as well, do you not see this as a problem? You told the future Duke to go to bed like a child. You are handling things he is supposed to be handling. You and your father are doing things behind his back, and not telling him. This makes no sense!
She isn't a better candidate either as time goes on because she jumps to conclusions, and seems over confident about those conclusions. Also, if the brother is the one who knows that mountain best, wouldn't it make sense for him to go? I do not know.