I have set up family sharing for IBooks with my wife. She has purchased ibooks as well, but she cannot see the books I have purchased. We each have our own Apple Id's, we have turned on family sharing on both devices.
We are trying to allow my wife to have access to my Ibooks library. We have set up family sharing on both devices. I have even changed her purchase user id to mine in the shared purchases area. No matter what we do, when we go to her library, there is no indication that she can access my library.