How can I stop my cat from urinating on the carpet?

My cat, Kirby, whom we've had as a kitten for almost 6 years now, is in danger of being taken back to the animal shelter. She keeps urinating in the same areas along the carpet against the wall, and my parents have tried just about everything to make her stop. We brought a dog into the house a few years before, and we took her back to the Humane Society after she started chasing the cat around the house, and nipped her tail. We brought another cat into the house named Morris, and Kirby practically beat him up whenever he tried to play with her. For some reason, Morris started urinating and pooping on the carpet, and we ended up taking him back. Then Kirby started doing it. Before the other animals arrived, Kirby never did anything on the carpet except for the occasional hair ball. Please help, my parents are really at the end of their rope.