Yes, bringing her sister in too would be a good idea.
Poor baby, I expect her legs are exhausted, she may have strained her muscles. I'm sure she spent a lot of time trying to get out. Once she's had a chance to rest, if she's still struggling to balance, there could be something else going...
Is she breathing normally, are her eyes open and alert?
I expect she's exhausted, I assume she became waterlogged?
You did the right thing by drying her off, I would keep her inside, so she can stay warm until she's feeling better.
Perhaps add some vitamins to her water (if you have nutri...