I'm not going to clean up after my fiances cats
Why do you not want to help her out?
She's not taking care of her cats properly.
Well, I understand your frustration then.  People should take care of their own pets.  If she's using you to do that instead, you're right to stand up for yourself.
The cats are basically a rat problem. I'm planning to banish them.
You definitely should not banish the cats. You are better off just letting your partner know that it is their job to take care of the cats. It would make her happy however for you to put forth the effort of cleaning up after the cats.