You can make your own OC, whether you are Ace's Little/Older Sister/Brother or Luffy's Older/Little Sister/Brother. Do whatever.

You Live with Ace, Luffy and Sabo with the Bandits that Garp has watching you guys. Dadan, a Strange Bandit with Orange Curly hair and doesn't show that she worries about you guys but actually does, is your guys Caretaker.