That's quite an experience to go through. You don't have any obligation to forgive your brother. You would be wise to get out of being in between your parents and your brother. If they will tolerate behavior from him that you think is disrespectful, that is really their business. It will probably be that way for as long as you all live, and you would save yourself a lot of aggravation by accepting that you can't change it.

Look at the bright side. If he and this new girlfriend get along, maybe he will be looking to move out soon and get a place with her. Sooner or later, you will probably be moving out of there, yourself. In the meantime, it is your parents' house, and it is up to them to set the rules, or lack of rules. It sounds like your parents are more disturbed by you and your brother fighting, than by your brother's behavior. Just ignore him and don't try to change what you cannot change.