...that the eldest unmarried adult daughter was referred to in the third person as Miss (Last Name), while her younger sisters were simply (First Name) (Last Name). Of course, when addressed directly, each daughter would be Miss (Last Name).

Come to think of it, I'm a little surprised that these anonymous friends of CW know so much about the Elliot family, but are unknown *to* them. I wouldn't expect them to be more than passing acquaintances, but I would expect the Elliots to know their names! Perhaps Elizabeth and Sir Walter are right in surmising that the whole world is interested in the Elliot family! ;-)