many children who have been through similar question whether they were to blame for what happened, that is natural behaviour, what is totally unnatural and illegal is what your brother did to you, you are in no way at all responsible for what he did.
mine did the same to me and yes i questionned whether it was my fault, blamed myself for years (it went on for years parallel to similar and emotional abuse from step father), but as i matured in adulthood i realised it was not to do with anything i had done or said, it was 100% their wanting to be pleasured, it could have been anyone, but i was there so they used me. the sad part is it is they who lost out in the long run because it made me so much more determined to achieve enough to look after myself so i would never have to rely on them in adulthood, and i did, i moved away and became the first person in my whole family line to ever own and run my own buisness, one they never got any rewards from. ok i hit rock bottom a few years ago but had the courage and determination not to go back there for support.
they do not know anything about my life now, they think i am still running my buisness, little do they know i am so disabled i can't even sit up without support! but the determination and courage are still shining through in me