I can’t make a diagnosis on the basis of a letter but your letter certainly sets off my inner alarms. Your whole family is under siege. You haven’t talked to him for 11 years even though you live in the same house? Things must be really bad!
Your brother sounds very troubled. I understand completely that, besides being afraid of him, your parents want to protect him and help him if they can. But it doesn’t help him to be allowed to continue to tyrannize the family. He isn’t getting treatment he probably needs and he isn’t learning how to be socially acceptable.
I urge you to investigate whether there is a domestic violence program in your city. Although most of the people who come to them are victims of spousal abuse, most such programs are also familiar with other variations of family violence. Counselors there will be able to give you valuable information about how to handle your brother and what resources are available to the whole family. They can also offer you and your parents important emotional support as you start trying to manage the situation differently.
No one should live under the constant threat of violence. I hope you will share your letter and this response with your parents. I very much hope they will seek out the practical advice and support your whole family desperately needs.
I wish you well.