I would keep checking for YC availability. You can also check for BC and BCV availability as they share SAB with YC. If that doesn't work, as PP pointed out, you could add them to your reservation for the day if you haven't hit guest capacity in your room. If adding your sister means there are now 3 adults in your room, there will be an extra adult charge for that night. Or, if all of that fails, the best thing to do, if you want to swim together, is go to one of the water parks. If you happen to have park hopper tickets, adding the water park option is not that much more.