Wow, your brother is nasty! Maybe he has some mental issues that he isn't aware of, or doesn't want to admit. He sounds like he might be bipolar as that might explain his rapid mood swings.

He sounds like he has some deep rooted resentment for your parents and yourself, and it sounds like he's jealous of you maybe thinking that you're the favorite, idk. Have you ever talked to him about why he treats you that way?

Tell him not talk to you like that and to not call you mental as calmly as possible. The next time he insults you, tell him to never talk to you like that again. Stand up to him. He is a bully it sounds like. If he refuses to stop, just ignore him and walk away and don't talk to him. Give him NO response. If he truly is a bully, then not giving him an emotional response is something you have to do. Bullies love getting a reaction out of their victims.

Cut him out of your life. He is toxic. Avoid him as much as possible. You need to stand up to yourself and demand respect and set clear boundaries.