All those little girls running around, Mrs. Bennet would have been welcoming to the helping hands of a responsible older child. Charlotte would have been around 8 years old when Mary was born.

It also would explain Mrs. Bennet's comment about how she raises her girls better than Lady Lucas:
"I always keep servants that can do their own work; my daughters are brought up differently. But everybody is to judge for themselves, and the Lucases are very good sort of girls, I assure you. "