I'm pretty sure that I read it here, but I don't remember what it's called and I haven't been able to find it.
Lizzy directs Mary to choose a different song to perform at the Netherfield Ball and she gets compliments and smiles and it makes her feel good so she approaches Lizzy the next morning to get more advice, etc. When she's at Hunsford, Mary is responsible for chaperoning Jane and Bingley when he proposes (is this the one where there's a fire in his warehouse and Caroline ends up at their aunt's in the north? I'd like to reread that one too), I don't recall what happens to Lydia, but Kitty becomes Miss Catherine and grows up a bit as well.