Xbox app says I have a game and after launching, says I need to buy it again

Did you actually purchase it or just install it through Game Pass? You can only access Game Pass games while you have a subscription and while the game is still on Game Pass. When your subscription ends, you can only play the game if you purchased it.

do you think that there is any way to play my xbox version on my PC so I don't have to buy it again?

I believe they may be different versions of the game, which would mean you would need to purchase the PC version.

theres a rather sizeable Minecraft FAQ that details all the different versions of the game, and which will work on what device

Hope that helps

Hi xMusicalMoodyx,

Thanks for replying! Currently, there are several different versions of Minecraft, as our Ambassador stated, and the way they are coded is different as well, which means that if you own the version for Xbox One console, it will be a different version
for a Windows 10 PC. This means that you will need to purchase the version of Minecraft for PC in order to play. The good news is that when you sign in with your Microsoft account, you will be able to access many of the same items and/or progress that you
have already established.

Minecraft - Overview

I hope this information helps a bit more!

You need to ask your brother and tell him to check his order history as well and check if the game transaction is visible under his account. Kindly ask him so we can proceed with the next step.