Hello, craigik, and thanks for asking your question on Just Answer,
Well, actually a lot depends on who is involved. When your sister took possession of the photo books, at that time she took responsibility for them. I do not see how anyone else could be held responsible for the books.
You mention that you have searched the house - did you live with your sister? Is that perhaps why you feel you may be felt to be responsible for the photo books?
Even if you lived in the same household, it was your sister who borrowed the books. You are not responsible in my opinion.
My best recommendation regarding this is to simply keep looking for the photo books. If they are found, great. If they are never found, that is unfortunate, but there is really no one at fault in their loss. Since your sister has passed away (my sympathies), there is really no one to hold accountable for the loss of the photo books.
I hope this helps relieve your mind a bit. If I can be of further assistance, please let me know. I hope you find the photo books!