What does it mean when you dream about your brother raping you?

Insecurities about your brother, I'd say. It could also be a fear of rape coupled with fear of being betrayed by those you hold dear. Then again, it could mean nothing; just an average nightmare. Sorry for inappropriate use of the Word "average."

In dreams, "rape" can represent personal violation of some sort. It need not be a sexual violation, but any inappropriate violation of boundaries. Also, people in dreams can symbolize someone or something else, with some similar characteristic. For example: Your "brother" might represent a worker from "Brothers Plumbing" who came into your home (violated boundaries) to fix the sink when you were not expecting them. Or it might represent your boss's brother giving your orders that violated the rules.

.... see the link below for more examples and further discussion; scroll halfway down the "Nightmares" page.