In this situation, you can file a Zero FIR where you can ask the police that if anything wrong happens with your father then your cousin will be responsible for the happening. If you want to set your family free from toxicity then you can hire a lawyer to distribute the property equally and take legal action on your property. Ask the lawyer to mention it clearly that no one is allowed to ask for your father’s property portions and only your father is the owner. Also, you can move out somewhere else if you still fear your cousin and shift somewhere safe.