Isabella is shown in these chapters to possess a dangerous vanity: emptiness of words.

Vain, of course, has an archaic meaning of being senseless or foolish (which we certainly see in some of IT's statements), but the Latin root is even worse; it means empty. IT not only obscures her meaning behind doubletalk ("Oh, don't tease me if you see a sallow complected man. Like your brother, whom you resemble so strongly. Who spent Christmas with us. DO YOU GET THE HINT YET, MY DEAREST FRIEND?"); she also uses doubletalk to say things that have almost no meaning ("Miss Andrews is the sweetest thing" becomes "Well, not really, since she's insipid.")

I think JA is depending on our own experiences to guide us to the conclusion that this type of vain talk has one of two consequences: we are either dealing with a person who will end up as comic relief, or who will later inhabit a less benign role. In either case, it's a foreshadowing of sorts.

Tim