In Your Media Library
, you could try to move your books into the trash can. They will disappear from list view, but are unfortunately still searchable, as the trashcan cannot be emptied. Very infuriating, actually!
I have been meaning to call amazon and ask if they can empty out my trashcan (there are 300 French classics in there that I downloaded for an aunt - ugh!), so you could try that, and, if they can't do that, try to create a separate account for yourself, at least for the books you don't want to advertise to your family, and use Calibre to add them to your library.
In geneal, if amazon finds out that one has more than one account, they may close them all out, so it is a bit tricky. Maybe if you call them, they have a different solution.
To me, the easiest would be to just let people delete their content for good if they so wish.
What I do not know is if the reading suggestions carry over between devices, which seems to also be your concern: say you use your mobile phone to browse and purchase fantasy ebooks - would someone sharing your account see fantasy titles suggested when they browse amazon from their PC? Or are the suggestions driven by local cookies, in which case even two separate accounts would share the same suggestions if you use the same device to browse their store.
In any of these cases, after completing your purchases, simply by browsing a category that is approved by, say, your grandmother, will help, since you'll end up with reading suggestions for other cook books or knitting books, or whatever else she may approve.
One more thing to take into account is: If you signed up to receive email suggestions: amazon knows what I read and periodically tells me that, because I bought book xyz, I may be interested in those similar books. Does someone other than you see these mails?
My reading was heavily monitored when I was a kid (no YA or children's books allowed, once I was in 5th or 6th grade), so I can totally relate to why someone would want freedom to read whatever they please. In my day, I had to hide the books of my choice in my library backpack underneath the approved titles.