Doomsday10159 Everyone's Apex progress is stored on the EA Account level.
So, if you and your bother both have a different platform account that's linked to the same EA Account, then both of your Apex progresses would of been merged together when Cross Progression was rolled out between October of 2023 and April of 2024.
Unfortunately as well, when a platform account is later unlink from an EA Account, all that does is stops the unlinked platform account from being able to access the Apex progress that is stored on the EA Account, (which also means that the unlinking of a platform account does not let you transfer, (or merge), your Apex progress with another EA Account).