Can obsessions related to childhood sexual abuse be removed, what is their effect on virginity, and how can one resume a normal life?

Your brother's actions, as you described, after he reached puberty, constitute a grave sin. You should avoid him as you would a stranger, and you are not held accountable for what occurred because you were a young girl at the time. As for the obsessive thoughts, repel them by ignoring them and remembering Allah. If the hymen is no longer intact, do not inform anyone about it. If your brother persists in his actions, then inform your parents so they may restrain him.