This is not typical. Usually you can add any iPod to any iTunes with no issues.
First check to see if it was really wiped. It may be that he used a different library than yours.
Edit->Preferences, Advanced should show you where his library currently is. If that is not correct you can close iTunes, hold the Shift key and start it back up and choose your library.
Second, you can restore your backup.
Third, you can look in Previous iTunes Library folder to see if due to an upgrade your brother did, a copy of your library is there. It will be named iTunes Library <date of upgrade>.itl. All you have to do is copy that file from there to your normal iTunes Library location, remove the date and start iTunes again.