Unless magic is a really niche elective, I would assume the (older?) sister would have a grasp of it too. Especially if the world is high magic reliant.
So like, both brother and sister from the same world, right? It one thing if they are middle age for the difference interest and background to develop, but they seem pretty young. Why does the sister not know how to solve the problem herself?
Old man Henderson short(with audio by StephanoRex)& full version:
Old Man Henderson
Rethinking about it, are there people that read trpg and not know 'old man Henderson'? I don't play trpg but have heard the story years ago & it sounds like something that learn as intro to plot derailment if...
He is not entirely enemies-less, being small & vulnerable not exactly something you want to announce. Plus he is demon king of pride, probably won't want other to know embarrassing stuff like that.