You should advise him to seek help from mental health professionals. It is difficult to know what is happening to him when he drinks and gets out of control. He may be drinking to cover up the voices you said he hears or it could be that the alcohol is part of the reason he hears voices. Either way, it is imperative that he seek some form of assistance.
Based on your letter, your brother has not been stable for most of his adult life and it seems to be getting worse. He is now at the point where he is acting out violently towards members of your family. This makes him dangerous.
Suggest to him (while he is sober) that he might benefit from mental health assistance. Perhaps, if the two of you are close (emotionally), you might be willing to go with him for support (i.e. drive him to the facility, help him choose where to go, etc.). If he has support he might be more willing to attend treatment. You and the rest of your family might go to him and request that he seek some sort of assistance. If the family approaches him with the same consistent message “please get help” this might be more powerful than you talking to him alone.
If he does not get help or he continues to drink, there may be little that you can do. You cannot force anyone to seek treatment. If he is a danger to you or your family you may have to resort to calling the authorities. This of course would be a measure of last resort but one that must be used if he places you or your family in danger.
This is a difficult problem to solve because it requires that your brother realize that he needs help. Do your best to assist him in getting help and keep trying until you feel that you have done all that you can. He is fortunate to have a sister who cares about his well-being. Thanks for writing and please write again if you have any follow-up questions.