I'm going to tell my brother to fuck off for pretending to have a more serious mental illness than he actually does
You think your brother is faking a mental illness?
Yes, it's true. Just the other day I overheard him talking to someone on txt. He was making fun of his disorder.
While I understand you are angry at your brother for faking a mental illness, I don't think you should resort to swearing. Maybe you can talk to your brother about why this is upsetting you.
I can't calmly talk to him before telling him to fuck off first.
Your anger is justified, but maybe express it in a less vitriolic way? All that anger will do is just prevent you from getting your point across.