When should I intervene when my rats fight?

I have three rats, two sisters from the same litter, and a younger female that I brought home in July/August. The introduction period went relatively smooth, I slowly introduced them to each other, and soon they were all happily living together in the same cage, with a few squabbles now and then to determine the pecking order. My problem is, five months later, the older girls have started ganging up on the other one. They've been having big fights, and her neck is covered in scabs. The rest of the time they get along wonderfully, they sleep together and groom each other. I'm starting to worry about the rat whose getting beat up. I don't want to separate her from her cagemates, but they're obviously being hostile toward her.