The family sharing has very careful wording.
I really don't think it is going to be that revolutionary of a feature. It is family, so whatever additional restrictions they have, I doubt you can belong to two different 'families'. I would bet money that it requires both parties to be online at all times, restricts the member using the shared library from running the same game that the sharer is running, and only one family member can be in your shared library at a time. So it is basically like ONE family member can grab a disk of yours and play it.
I really don't think it is going to be that revolutionary of a feature. It is family, so whatever additional restrictions they have, I doubt you can belong to two different 'families'. I would bet money that it requires both parties to be online at all times, restricts the member using the shared library from running the same game that the sharer is running, and only one family member can be in your shared library at a time. So it is basically like ONE family member can grab a disk of yours and play it.