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. She kind of reminds us of that amazing line from A Fish Called Wanda: "You can't call me a monkey – monkeys don't read philosophy!" "They do, they just don't understand it."
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 flies right.