Hmm, Cataholic and Mugsy are right, some dogs just won't get along. Dazzi has a few that she absolutely dislikes at the dog park and when they come, I either put her on a leash or put her in the time out pen. Usually after she has a fight with one, she ignors it after that. But there are a few....

With dogs we see on a regular basis though, like Amy's, we both let them know that fighting is not acceptable. And if there is a thing that they are both aggressive over, we try to make sure that thing (stuffed toy or food) is not around to be an issue. Jack and Spot don't care much for each other but so far they have just stayed away from each other. How old are the dogs involved? What does your sister do when this happens? The easiest solution is to keep them away from each other. But there may be other ways to control the situation. It probably depends on your sister's willingness to try to work things out with them.

But just because Katie doesn't get along with some dogs does not mean she won't be okay with others. Dazzi does not go to many dog park gatherings because she is better with a few dogs that she knows well than lots who she doesn't know so well. Plus she thinks she is the boss at the dog park so that can be a big problem with new dogs.