If the indoor kitties aren't neutered, that could be part of the problem. If the outdoor kitty doesn't come in that often, she could be upset and taking it out on the indoor kitties. Whenever I've had cats that didn't get along, I kept the troublemaker isolated and gradually introduced them to each other, just a little bit at a time. However, if you let her go back outside then you'll probably have the same problem all over again. Cats like consistency; they hate change. Once she comes inside and becomes docile, it's a good idea to keep her in permanently. Good luck w/this situation. I know it's tough when cat fights occur.