If I understand correctly, your brother is part of the family group but is not able to use Apple Music. This can happen for a couple of reasons. One reason could be that the organizer of the family group has an Individual Membership instead of a Family Membership. Another could be that iCloud Music Library is not enabled on your brother's device. Or there is a connectivity issue with the device that is not allowing them to connect or download. I would recommend you read these articles and use them to help troubleshoot the issue.
If you're already an Apple Music member
If you have an Individual Membership to Apple Music, you can change to a Family Membership in the Music app or in iTunes on a computer. Learn how to manage your Apple Music membership on your iPhone, iPad, iPod touch, Mac, or PC, or on an Android phone.
Get an Apple Music Family Membership
On your iPhone, iPad, or iPod touch
Go to Settings > Music, and tap iCloud Music Library to turn it on.
If you can't download, play, or access songs
First check iTunes to see if the song is ineligible for upload. Additionally, if you added the song to your library from the Apple Music catalog, it might no longer be available on Apple Music.
Check your Internet connection
Open Safari on your iPhone, iPad, iPod touch, or Mac, or a web browser on your PC, and go to any website to make sure you have a strong Wi-Fi or Internet connection.
If you can't connect to a Wi-Fi network on your iOS device, you can try using cellular data:
- Go to Settings > App and iTunes Stores.
- Turn on Use Cellular Data.
You might have to pay a fee to download over a cellular network. Contact your wireless carrier for details, and learn about cellular data settings and usage.Get help with iCloud Music Library
Thank you for using Apple Support Communities. Have a good one.