I'm looking to unify the family's iDevices under a single App Store ID, but if I do that, does the app data for existing apps get erased? My brother would probably be angry if his angry birds progress disappeared...
No, you do not have to repurchase the application.
However if that device user wants update, he will need to either purchase the app himself or use your account again to update.
Honestly I've never done a re-purchase [with a new account] to get an app update before, so for this I am unsure if it will require deleting the old data. I think the best way to get updates is to use to the old account login. Otherwise new purchases can use the new account.