You're 18, but you do still live at home with brother, correct?

Do you think you could talk to your mum and explain that even though they say not to hit you, he continues? One things that might help you feel more secure is a safety plan--like, "if he starts yelling, I am allowed to (leave the house, lock myself in a safe room, call mum and dad) and if he hits me, his consequence will be ____________ (lose car privileges, etc.). It's unfortunate that you have to be the one to ask your parents to enforce basic boundaries like your physical safety.

I think if you get your feelings about this out in your family so everyone knows how it affects you, you will feel relieved--even if they don't respond as positively as we might hope. You have a right to express your fears, and you have a right to be protected.

It might also help if you set boundaries for yourself, too. Like, "I will (stay at a friend's for the night, call the police, etc.) if he hits me again." Having a plan for what to do can help me feel more in control and less afraid.