Why doesn’t apple want to refund something I didn’t buy

They do help and they also do refund, but that is if you make a mistake once or twice-not very often as your case seems to be.

i suggest set up a password so that your brother cannot make accidental purchases in the future.

Read this:

https://support.apple.com/en-us/HT204030

It is your responsibility to make sure your brother doesn't buy apps. Apple has nothing to do with that. Take responsibility yourself. Every-time your brother buys an app uses it, and you expect apple to refund you. Wow!