Hi clover, welcome to PC. Firstly, no one here can diagnose you, we are not professionals.

I am sorry your brother is scaring you that way and you witnessed the violence between your brother and your father. PTSD can happen if one is traumatized and genuinely believes their life is in danger.

I suggest that you see your school guidance counselor, even visit the nurse and explain what you have been experiencing. If you are afraid to talk about it, write it down like you did here and give it to the nurse, guidance counselor, or even if you have a teacher you like and trust.

It's important that you get help with this, you are too young to deal with this challenge all by yourself.