When I was in grade one I had a adopted brother and he would tell me that it was okay and that his friends at school did the same thing with their sisters. I don't remmeber much but when I got to grade 4 I learned what a period was and I told him I started it and he stopped. When he got kicked out of the house he told me if I told anybody I would be taken out of the house like him. I didn't know when I was that young and I told him I didn't want to. Would it be considered rape?

It could be yes but the fact that you were a child is far worse than just a statutory rape charge. How old was he at time of the incident? That's very important info.

Statutory Rape is having sexual intimacy with someone under the age of consent.

Yes but there is a different between a minor and a child. Also, if the guy was over the age of consent himself or also a child will matter. He may or may not be charged as an adult.

All this chit-chat is nice and all, but these are the questions which must be answered before OP's original question can be properly answered:

"It" is sexual activity?
What state or country did this occur in?
How old was he?
How long ago did this happen?

Yes, it may have just been touching ( still wrong of course) was he in 4th grade or 10th grade, not enough info to give a legal answer, Moral answer it was wrong and you need to get into theropy.

There were two questions asked here. The title asks what is statutory rape. The body asks if what happened could be considered statutory rape.

I answered the title. But to determine whether a prosecutable crime was committed we need more details about what the brother actually did. Clearly the OP was underage.

But whether the brother could be prosecuted depends on more information. But, as Chuck recommended, the OP should be seeing a counselor to deal with this.