I'm angry and blame my brother's girlfriend for my mental health decline.
Why would you blame your brother's girlfriend for the state of your mental health?
I think she brain-washed my brother and I get these visions about him that would never happen in the real world.
I'm sorry you are feeling that way about your family. Starting fights with a sibling's partner will create conflict between you and your sibling. I think you should try talking to him about things and how you are feeling.
SHUT THE FUCK UP, YOU KNOW NOTHING ABOUT THE REAL WORLD
I understand it can be frustrating to not receive the response you were hoping for. I'm just pointing out to you that if you approach things from a certain manner they will become worse for not just your relationship, but your mental health. If you feel this anxious about their relationship you may want to seek some professional help.