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Mary and Kitty are the other Bennet sisters, neither of whom are super-developed as characters. Honestly, it's probably for the best, since there are enough Bennets in this novel already.
Mary is a caricature of an overly-bookish young woman, who reads and memorizes a lot, but doesn't really get the point of what she is reading and is totally unable to think critically about her books. Like, look at her incredibly boneheaded response to Lydia's elopement:
"Unhappy as the event must be for Lydia, we may draw from it this useful lesson: that loss of virtue in a female is irretrievable; that one false step involves her in endless ruin; that her reputation is no less brittle than it is beautiful; and that she cannot be too much guarded in her behaviour towards the undeserving of the other sex." (47.44)
Um, that's your sister you're talking about, Mary. Maybe show a little compassion?
Meanwhile, Kitty has even less definite qualities. She's basically like Lydia, just a little less so. Eventually, under the good influence of Elizabeth and Darcy, she straightens up and becomes "less irritable, less ignorant, and less insipid" (61.4). It's not much, but at least it's a start.