It had not occurred to me before, but of course Mary and Kitty are mirroring the behavior they've observed in their parents.
It also strikes me how ill-prepared the younger girls are for marriage, even though their mother keeps harping on how they need to find husbands to support them. We've now seen two good examples of young women who can run a household -- Jane and Charlotte. Mary and Kitty aren't stepping in to help, and it could be that they have no idea what needs to be done around the house. They just disappear in their rooms for the day and then show up for dinner. It makes me wonder what would have happened at home if Jane had also been invited on the trip with the Gardiners.