Redirected aggression

I just heard the saddest story this weekend. Two of our friends have these sister cats who used to be best friends. It all changed when the neighbor dog was in the yard and one cat Anna snuck up on the other Nala. Anna turned and attacked Nala. Ever since then Anna has redirected all her aggression and frustration to Nala. My friends have been working on behavior modification and so far it seems to be working Anna is getting less unpredictable to Nala, and Nala is standing up for herself. The really strange thing is that they still will curl up and sleep together. So I guess this is not as rare a thing as I had thought.