It sounds to me like this started out as "playing doctor," your brother wanted to know about the opposite sex and you were his only opportunity to do so. As you both got into your teens and puberty, then it became more wholly sexual in nature and was additionally fueled by hormones. I don't think it was technically abuse, since you were both "equal" in power/children (would have been abuse if it had been one of your parents). I'm very glad you became less isolated and went to high school and made other friends.

Did you ever mention it to one of your parents? I have three older brothers (was the only girl) and didn't put up with rough/rude treatment without complaining to my mother!