between elder and younger Bennets girls strikes. Someone in another discussion very reasonably said that Austen often gives us fine characters brought up in not too fine families, without any explanation why. So we have to just take it for granted.

I copy here my latest post in the P&P board, it is on Mr B but also concerns his daughters' formation:
I always thought if there was a time in the beginning of the Bennets' marriage life when Mr B still tried to make things right. His eldest daughters having grown up sensible and virtous confirms this idea. Then he must have given up at last and three other girls were left to their mother care solely and the mother herself to her own deficient wits.