I find it telling that Austen hammers away at the silliness of the two younger sisters.

In Chapter 7 alone (Volume I, Norton Critical Edition), she mentions it at least three times:

Narrator: The two youngest of the family, Catherine and Lydia....their minds were more vacant than their sisters'...

Mr. Bennet: From all that I can collect by your manner of talking, you must be two of the silliest girls in the country.

Mr. Bennet to Mrs. Bennet: ...I had hoped that our sentiments coincided in every particular, but I must so far differ from you as to think our two youngest daughters uncommonly foolish.

Clearly, the Bennets have given up on their two younger daughters, and there appears to be no effort on their part to change or improve their silly selves.