How do I fix this problem?

My cat, Mitzi, has been bullied by the other cats (mainly her sister) for most of her life. I don't understand why. I know that if she would just start taking up for herself they would start leaving her alone. Other cats have successfully done that. She finally had enough, I guess; because she has moved a couple of houses down the road to my brother's house (odd how she chose that house - did she know?) and he takes care of her most of the time now. I go up to his house to see her. She tries to come home sometimes and the other cats just run her off again. I know she wants to be home, but I just don't know what to do about it. She isn't the only one being bullied right now, but she is the only one that has seemed to move away (thankfully not far). They are also bullying Iddie Bit. She stays in the woods behind our house most of the time, but I can always find her and she always comes when I call her. As long as I stay with them the other cats leave them alone. But I can't be there 24 hours a day. What should I do?

Hi Lisa, I moved you to Behavior so that others who might have had this situation can help you out.

Are all your cats spayed and neutered? That would be my first question/concern. Are they inside/outside kitties? If you don't have a lot of land for 7 cats to roam, it is a given that they will be getting on each other's nerves. Each cat needs its own individual space and territorial disputes are common for cats in close quarters.

All of my cats have been spayed and neutered. They all come in and go out of the house when they want to (we let them in and out). And, yes, we have a pretty big yard, so there is plenty of room for them to roam around and find their own space. Thumper is the main bully. Sometimes a couple of the other cats see her doing that and they try to copy her (the cats that have stood up to her are no longer being bullied). Not all of them are bullying. The main ones are Thumper and Tater Dude, and now Rat has started guarding the back steps. She is like a guard kitty or something sitting by the back door. She only let certain cats go past her. I just wish that they would all get along. I want Mitzi to be able to stay home and not get chased or anything; and Iddie Bit to be able to start staying closer to the house without being chased back to the woods.

Have you looked into Bach's floral remedies? I've never tried them myself, but a lot of people swear by them. If you start a post asking about them, there are probably several people here who could recommend particular remedies for the bullies and others for Mitzi.
I have seen a cat "rehome" himself because he was being bullied. I found three kittens while visiting in the US a few years ago. My mom kept the male, and the two females were adopted by two different neighbors. After about a year, one of the females decided she wanted to move into my mom's house (she liked the dogs). She bullied her brother so much that after a few months he went to live with his sister's original owner.