It would bother me too if I were in your situation, and it is perfectly reasonable to ask your brother to be respectful of you and of your parents' home. He did wrong in laying hands on you. I have people in domestic violence groups (I'm a therapist and lead those groups) who were court-ordered for similar things. Your parents have created this monster and you can express your opinion and tell them that you don't like living with him acting the way that he does. If they would go and see your psychologist with you for a family session, maybe with just your mom once and then go from there, that could be helpful. But they are who they are, and your brother is who he is, and you can't change that. They are going to make their own choices and all that you can do is make requests.

I think that moving out will probably be your best bet. Then you can be the independent one who is responsible and takes care of herself, and your brother can be a user if he wants to and your parents keep letting him, but people who know you will see the difference. On forgiveness, that isn't for your brother. You should not let him continue to threaten and abuse you, and you need to do what is necessary to set limits and get yourself into an environment that you feel is appropriate. But staying angry at him will only amuse him, make your mom worry, and keep you miserable. For your own happiness and well-being, you owe it to yourself to let go and move on with your life. Have you heard of radical acceptance? It is the idea that even though you don't like or agree with what is happening, you know that it is the way things are, choose to accept that, and do whatever you need to do to take care of yourself. It's not easy, but it is the only thing that is going to work when you are dealing with people like this. I'm sorry that they are like that. Hang in there. You can change this dynamic for yourself and for your future family if you have one.