"From all that I can collect by your manner of talking, you must be two of the silliest girls in the country. I have suspected it some time, but I am now convinced."

Catherine was disconcerted, and made no answer; but Lydia, with perfect indifference, continued to express her admiration of Captain Carter, and her hope of seeing him in the course of the day, as he was going the next morning to London. (Chapter 7)

I never noticed before this reread that Kitty was disconcerted by her father calling them the silliest girls in the country. While JA and he usually package Kitty and Lydia together, JA separates them by their reactions to what he said this one time. Kitty is only mentioned once on her own account up to Chapter 9—the nerve tearing coughing in Chapter 2. Lydia is mentioned on her own account, without Kitty, twice by Mrs. Bennet’s preference—in Chapter 1 when she says Lizzy is not half so good-humored as Lydia and in Chapter 2 when she suggests Bingley will dance with her at the assembly ball although she is the youngest.

Lydia was a stout, well-grown girl of fifteen, with a fine complexion and good-humoured countenance; a favourite with her mother, whose affection had brought her into public at an early age. She had high animal spirits, and a sort of natural self-consequence, which the attentions of the officers, to whom her uncle's good dinners and her own easy manners recommended her, had increased into assurance. (Chapter 7)

In Chapter 2 Kitty fretfully asserts coughing is not self-amusing but Lydia is immune to reprimand and insults alike. The description of Lydia in Chapter 7 is wonderful; it gives a perfect picture of a thoughtless but amiable girl with more confidence than sense. Her confidence, sometimes egged along by only the merit of being taller than her sisters makes her unafraid of dancing with an unknown gentleman of consequence, probably of more consequence than she has ever encountered before in Chapter 2 or obtaining a promise for a ball from the same gentleman in Chapter 7. I think although Kitty and Lydia are companions much alike in their frippery pursuits, as early as Chapter 2 they are shown to be girls of slightly different calibers. ;D