It’s true that siblings fight. It’s true that most families experience conflict now and then. But not to the point where a younger sister is afraid of her brother; not to the point where a young man is abusive to his parents and his sister. What you are describing is not a “brotherly” thing. What you are describing is bullying.
Your brother seems to be more in control of himself than you think. If he can be civil to other people outside of the house, he could do the same at home. He is choosing not to. It may be that he has learned that all he has to do is act threatening and he can do what he pleases at home. Other people won’t tolerate it.
There is no good reason for you and your folks to put up with this behavior. At 20, he is old enough to leave home and take care of himself. The condition for remaining in the house should be the same kind and polite behavior he uses outside. Family members who love him should get his best self, not his worst.
If your parents can’t draw a line on his behavior, I hope they will consider seeing a counselor to help them feel okay about making reasonable demands on their unreasonable son. No one should live in fear in their own home. All of you deserve a peaceful and loving atmosphere at home. Your brother will only benefit from learning how to treat the people closest to him with respect and love.
I wish you well.