My wife accidentally started a new family share as the organizer instead of just accepting the invite to the established one. You can only be in one family share at a time. How can she delete hers, which she's the only member, and join ours?
She should be able to do it via Settings > iCloud > Family > Stop Family Sharing on an iOS device, or if using a Mac with Yosemite on it System Preferences > iCloud > Manage Family : Leave Family Sharing.