She is just far more capable than MC in every aspect, and has shows that she surpasses him for 5 years straight, in other words, she's smarter, stronger, better at politics, ect, and they would 100% be trained on stuff like that at a very young age due to being nobles, in other words, as an heir, she's just so much more capable than him that it's obvious that she is a superior heir to their household
The only problem, is that she's a shit person, which is the only thing that stops her from being a better heir, because she'll bring her house to her doom, which I did not think I would have to elaborate on