Hello Imnotanisheep,
It sounds like you're a great sibling for helping your brother with his HomePod mini setup. Let's see if we can help get it set up for him!
The person that originally set up the HomePod on their iOS or iPadOS device will need to make the changes to the HomePod. You will also need to be signed in to the same Apple account in iCloud and Media & Services on your device. If you were the person that set up the HomePod initially, this would be your device. You don't need to log out or log in with his account for these steps.
- First, make sure you have invited your brother to control accessories in the Home app.
Invite others to control accessories
1. On the Home tab, tap 
, tap Home Settings, then tap Invite People.
2. Tap 
to choose people with an Apple ID from your contacts list, or enter their Apple ID email addresses in the To field.
3. Tap Send Invite.
4. Ask the invitee to do one of the following:
* In the notification: (iOS or iPadOS device) Tap Accept.
* In the Home app: (iOS or iPadOS device) Tap , tap Home Settings, then tap their name.
* On Apple TV: Tap Show Me on Apple TV, then turn on one or more Apple TVs.
Allow others to control accessories in your home - Apple Support
Once you see your brother in Home Settings > People, you can set him as the Primary User for his HomePod.
- From the Home app on the device and account that was used to set up the HomePod, you should see your brother's name in the list. Just tap his name to assign him as the Primary User.
1. In the Home app on your iOS or iPadOS device, tap the HomePod.
2. Scroll down, tap 
, then do any of the following:
* Change the default account for music and podcasts: Tap Primary User, then choose which account should be used when someone’s voice isn’t recognized.
Change HomePod settings - Apple Support
All Our Best.
