It's a kid fic, based on Parent Trap. For this prompt:
Sherlock and John were married, had two beautiful twin girls and then everything went tragically awry and they separated. Years later, the girls meet at a summer camp and piece together what happened. They switch places for their trip home, hijinks ensue.
It's still a WIP so if that puts you off rest assured I will be cleaning it up at some point and posting it here. Otherwise I am updating every other day so you don't have to wait too long and if I miss a day you can prod me right here or over on Twitter and I'll try and get my arse into gear.