Why would my puppies attack their baby sister?

We had a dog that had 9 puppies, we found homes for 4 of them and still have 5 after 2 1/2 months. 3 of the puppies will attack the smallest little girl puppy. I've had puppies before and they are NOT playing, they made her bleed and she was walking with a limp and has many lacerations. I had to pull one puppy off her and her teeth were dug into her. We bring her in and she just cries constantly and wants to be out with them. Why would they do this?