I have a "paid" custom field (it could really be called anything, and most things are better than what I picked) that I use to mark books that shouldn't be shared, to distinguish them from PD/shareable/demo books that can be. When I import new books, I set that field if necessary as part of my general tag cleanup. Then, as our quacking friend says, use a
not filter to skip the non-shareable books, and that will only show the shareable ones. It takes some time to set up, true, but if you pick, say, 100 books and do those, that should keep your sister until you get the next batch done.
