I guess I framed things unclearly, still thinking of everything haha. I meant to say the sibling will be there along with X throughout. But X will be seen as the main character.

When I plan on transitioning the sibling as the main character, they are already known to the reader but with a different personality and new goal. They influence the rest of the story...and when they appear (maybe sporadically until the climax or maybe just a lack of POV), it's a big deal. There's tension, excitement. They're still mentioned throughout and drive the events when they're not appearing. My story will have many "main characters" but the sibling will be the most important one.

X will be part of the story and events but won't be seen as the hero anymore as you mentioned.

A main character set up as 'the chosen one' that turns out to be a sidekick could work alright. But only, I think, if the story that was being told was that story. The story of someone trying to make a difference but having to learn how to deal with someone else being bigger, better, and more important than them - that could be compelling.

Yes, this is what I'm going for!