I had been using different accounts to connect to VC from each client, although the perms should be no different. To make sure, I just now tried switching the accounts, but the behavior stayed with the client rather than following the account used to connect VC. (In other words, I'd been using account1 to connect my desktop to VC and account2 to connect client2. Just now I tried using account2 from my desktop and account1 from client2; my desktop still fails to retrieve GuestFamily while client2 continues to return that property.)
From client2, PowerCLI retrieves GuestFamily for all vms I've tried. From my desktop, PowerCLI does not return GuestFamily for any vm. Naturally, I did try many of the same vms from both clients.