Switching protagonist feels weird to me but I want the shift to work?

So one of my projects (of a set) involves the main character for a good portion of the story as the protagonist, then it shifts part way through to their brother and becomes all about him making the original protagonist a "side-protagonist" kind of. Feels kind of cheap to me in a way because it's like I'm favoring the brother over the starting protagonist then randomly sidelining the protagonist to make it all about this other person who then takes over the narrative, if you know what I mean? Like even in the sequel story it's about all their children with his child being the MC.

Context/Premise: As the story begins it starts with the sister, Ori, becoming enrolled in this prestigious military-like academy and how her and her group of friends basically try to become heroes (Think Fire Emblem 3 houses, RWBY, etc.). But unbeknownst to her, her older brother whom she hasn't seen since their entire family was slaughtered by him when they were children, due to the BBEG controlling his mind with his powers. He thought she was dead and she basically repressed it all, the story progresses and they ultimately come to blows with how she's been haunted by his presence in the back of her mind because of the trauma finally allowing her to gain her powers in the process of trying to kill him.

Effectively for the most part ending her arc in the narrative I guess? Then I have the story shift to have the brother basically take over the rest of the narrative till the end basically. There's more to it but that's the gist of their arcs in the story and how they switch. I want her to be the protagonist first but I feel weird about it.