My brother had pandora premium and I just got pandora premium family. I invited him to join my plan after he cancelled his subscription. It appears he can’t join my family plan until his individual premium subscription expires(please tell me if I’m wrong.) but the main question on hand is if he’ll lose his downloaded music by being forced to let his premium subscription expire before getting on my plan. Someone please help!
Sounds like your brother may be upgraded through a third-party vendor like Google Play or Apple. At this time it's not currently possible to join or upgrade to a Family Plan from these vendors. Because he has to wait for his subscription to end, unfortunately, this will remove is offline content and he will have to redownload the content when he joins your family plan. (Sorry about that!)
We do have a feature request to restore offline downloads after reinstalling or resubscribing. If this is something you'd like to see on Pandora, I would recommend upvoting this idea - this helps us know which features our listeners are wanting to see.