The third and fourth Bennet sisters don't get much to do here. Well, except fight off zombie hordes, of course—but we're talking character development here.
It's pretty much the same in the original novel. Mary talks a lot about stuff she's read in books, and that's about it—though in this version she does get to threaten Mr. Collins with a fork after he insults the Bennet sisters' by implying they actually cooked dinner.
How dare he?
Kitty is like a lighter version of Lydia, but she's a less flirty, so it makes sense that when Lydia is off taking care of her poor, lame husband, Kitty starts up with her training again:
Kitty, to her very material advantage, spent the chief of her time with her two elder sisters. In society so superior to what she had generally known, her improvement was great. She was not of so ungovernable a temper as Lydia; and, removed from the influence of Lydia's example, she became, by proper attention and management, less irritable, less ignorant, and less insipid. When she announced that she should like to return to Shaolin, for two or three years, in hopes of becoming as fine a warrior as Elizabeth, Mr. Darcy was only too happy to pay for the whole. (61.5)
Basically, there's hope for these little Bennet warriors yet.