Lydia has eloped, Mr. Bennet has gone to London to find her, Mrs. B. is putting on a great show of being helpless and hysterical, and poor Jane is left to run the household and wait on her mother. So where are Mary and Kitty? On the day Lizzy and the Gardiners speed back to Longbourn, the Silent Sisters don't even appear to greet them until dinner, because they "had been too busily engaged in their separate apartments...One came from her books, and the other from her toilette." Sweet Jane says she is sure they would have helped, but Kitty is slight and delicate, and Mary needs to rest from studying so much.
As a result, neither one of them does so much as bring Mom a cup of tea. They've figured out that Jane will do it all without complaint -- I doubt that they'd get away with that if Lizzy were there the whole time.
I mean, really, who is Kitty getting all dolled up for? It's not likely she's leaving the house anytime soon, because the family can't really show their faces in public, and her partner in crime Lydia isn't there. And she's not too delicate to go traipsing all around Meryton after officers, is she?
As for Mary, she needs to REST from reading all day? What's she studying for, the bar exam? Both of them need to get out of their rooms and help out!