This may be an issue with the Apple Music app on Android rather than the Motorola phone itself. Apple does support managing an Apple Music Family subscription on Android.
Try these steps:
- Update Apple Music through the Google Play Store, then restart the Motorola phone.
- Open Apple Music and go to More -> Account -> Manage Family -> Add Family Member and try again.
- Check that everyone is using an Apple Account with the same home country or region, and that your sibling has an active Family subscription.
If it still errors, try managing Family Sharing from an Apple device or another device’s browser, if available.
More info: Use Family Sharing with Apple Music on Android.