Before fighting the second Big Sister (if you are harvesting ADAM but saving the girls), you will get an Enrage Plasmid. There are two bodies to scrounge ADAM from. One is more easily securable, which has a security camera, a rocket launcher bot, and a large puddle of water in this area. The Big Daddy comes back several times. If you drop off the little girl in her hole near here, you can wait for the Big Sister to show up. Then, enrage the Big Daddy so that he takes on and kills the Big Sister. Your only concern will be Splicers and the Big Daddy after he is done with the Big Sister.