Help with buying a game that i already have downloaded.

I have minecraft bedrock installed on my phone, but i didnt buy it with my account, I used my brother's to download it (since he had already bought mcpe), but now, when i try to play, play store says i need to buy the game, and i cant do that because the "buy" button wont show up.What do I do? (Samsung A54 5G, fully updated).