Ok, I was playing through a level and I didn't have much health or any amo so I took over a big daddy to help me out. The little sister scattered to some hiddey-hole and I killed the big daddy. I went back to find the little sister but couldn't. I spent about 45 minutes looking for her and never found her so I went on to the next level. What happens? Do I still get the achievement for saving all the little sisters or do I have to play through the entire game again just to get her?