I believe that you are right, and Cassandra didn't destroy the letters. Not until this group read, however, had I any reason to believe that it was not the case.

Reading these letters is the equivalent of snooping in my opinion. They are illuminating on so many levels -- understanding JA's wit in "real life", her character including an evil streak, and her life and times. But I can't help wonder how she'd feel if she knew that her personal correspondence to her sister had been laid out for all the world to see.