Yes. You can create a new Apple ID for the Mac and use that to join your brother’s Family Group.
Do note, purchases can never be merged with another Apple ID and will be forever tied to the Apple ID that made them.
You will need an email address that has not been used with an Apple ID previously in any capacity.
Follow the steps here: link-> How to create a new Apple ID - Apple Support
You will need to sign out of the App Store, iCloud, Messages and Facetime on the iMac, and sign in with the new Apple ID.
Link-> Sign in with your Apple ID - Apple Support