Hello Ghoixt,
We'd double check that you're still not part of that Family Sharing group with these steps: Leave Family Sharing - Apple Support
Remove your account from a family group
If your Apple ID is being used to share purchases with a family group that you no longer want to be part of, you can visit your Apple ID account page and remove the account from the family directly.
Go to appleid.apple.com and sign in with the Apple ID and password for the account you want to remove.
In the Family Sharing section:
1. On your iPhone, iPad, iPod touch or Mac, sign in to appleid.apple.com with your Apple ID and password.
2. On your iPhone or iPod touch, tap the downward-facing arrow at the top of the page, then tap Family Sharing. On your iPad or Mac, click Family Sharing on the left side of the page.
3. Tap or click Disconnect Account.
4. Tap or click Disconnect.
After you leave the family group, you can start your own family group or join another one.
In case you need to change or update your billing details, see steps in the following to accomplish that: If you need to change or update your Apple ID payment method - Apple Support
Should you continue to have issues, then we'd share the resource above with your brother, since it provides steps he can take to remove your account from Family Sharing.
In case you continue to need support, then it would be best to contact Apple directly: Get Support
They'll need to take it from here.
Wishing you well, take care.
