Think about it from your character’s perspective.
You need to give them a strong drive. Beyond family business what makes the MC need to be the heir. Beyond sibling rivalry. Is it power that motivates them? Why can they absolutely not be second in command? Before anything else your character HAS to have strong motivation.
Without motivation there is no reason to need to be the heir.
Now, treat it as a monarchy. This monarchy means that when running well you have access to everything you want. Power, money, infamy, everything. Only, to remain the most powerful in the monarchy and get the most/best resources you need to, kill traitors, silence opposing speakers, and use your influence to make every word be unwritten.
Your characters HAVE to understand this.
When they do that means they are willing to do everything mentioned above to be the best. That means instead of making friends and dating they make connections with higher ups, politicians, lawyers, judges, everyone they need if something goes wrong. Instead of studying homework they’re busy learning how to use weapons. Instead of getting pizza, as so many stories do, they have to smuggle in illegal items.
This means that instead of joking and being clumsy, they should be able to formulate detailed plans to reach their goals.