bipolar cat

We have a cat, Cleo. She used to be a loving and playful cat but that has all changed. My parents adopted 2 Maine Coon cats a few years back, one of whom has already passed away. Ever since they came home, Cleo has abandoned us and behaves really strangely. She still comes in at night and to eat occasionally, but if you acknowledge her, she growls and hisses. I try to pet her, and she arches her back and doesn't seem to mind, but growls ferociously the whole time. She has never scratched anyone, but acts like she hates us all the time. If you say her name she growls and spits. I think she still wants affection because she sometimes pauses when I call her like she is torn about the situation but I don't know how to win her back. This has been going on for about a year now and I am afraid that it may be too late for her. Sometimes I pick her up and although she makes lots of angry noises, she doesn't really struggle for the first few minutes. I don't know what to do and I really want our cat back.

Behavior changes can sometimes mean something is wrong physically. Has she been to the vet and been checked?

Hi and welcome to TCS! My first thought is this isn't bipolar behavior as I'm sure you know. Most important is a clean bill of health, it is very common for behavioral changes if ill or suffering discomfort from ailment. Also Maine Coon cats are known for being very easy going, family oriented cats. It would be best to try and think of how her introduction was to the other cats, was she given less attention? Where they introduced slowly to her territory or was it free for all? She may have felt punished and unloved if they suddenly had the rights and attention to every thing and from every one that she use to have.