I am so very glad you wrote. Please don’t feel guilty about your reaction. It is very common for people who are being assaulted to freeze. This is especially true when the abuser is bigger and stronger and has already shown that he is capable of physical aggression. You can’t fight him. All you can do is detach from yourself.
Make no mistake: What your brother is doing is a crime. He is sexually abusing you. It is not normal. It is not okay for him to take advantage of you if you have to sleep in the same room. He continues assaulting you because he can get away with it. He has already tried to have sex with you and will do so again. Stop it now before it gets even worse.
You really do have to tell your parents. I’m sure they won’t want to believe it. No parent wants to believe that their son could actually sexually abuse his sister. It is understandable that they would rather think you are crazy. For some parents, crazy is better than criminal. But that doesn’t make their reaction okay. They need to protect you. In fact, they need to protect both their children: You from being raped by your brother; your brother from going to jail.
Take care of yourself. Before you have the conversation, arrange for a friend to be with you to give you support. Write out what you want to tell them. If words fail you, you can hand them your letter. Do not have the conversation with your brother present. He will of course deny it and try to make it look like it is your fault. You don’t need to deal with that.
If your parents call you names or try to make you retract your statement, you need to stay calm and simply tell them they have to stop it or you will go to the police. If your parents persist in not believing you, then go to the police and stay with your friend until your parents send your brother away. You cannot safely live in the same house with him. He is likely to try to get even with you for telling.
Abusers can only continue abuse as long as everyone keeps their secret. You have a right to be furious. You have a right to protect yourself! If your parents handle this as they should, there is no need for you to have a negative reputation. Even if the story does get out, you can hold your head up with pride because you refused to be victimized.
I wish you well.