It is quite a lot to handle on your own. I would find a therapist and be upfront and specific about what you are requesting. Thinking in terms of normal or abnormal is useless, because it does not define your goals and even less so a path towards achieving them. Do you want to be able to have orgasms without thinking of the brother raping you? then that would be your goal. How about an incremental approach - say, if you move away from thinking only about him raping you but think of those stories of incest that, as you wrote, turn you on? That would be a moderate amount of success, I would venture, because you "repertoire" would be expanded. From there, perhaps, you can move completely away from incest, or perhaps not due to deep imprinting, but it is worth exploring.

Sexual molestation by older siblings is common; the therapists whom you would interview will undoubtedly have heard many stories of incest, both in the course of their studies and in the course of practicing psychotherapy. There is nothing shameful about what you are describing and not telling anyone is for sure (!!!) counterproductive. If you tell, you might be surprised to have the problem go away by itself, simply because you finally talked about it.