I'm 15 and my brother is 20, since you have to be 18 to get the information about your parents, would my brother be able to help me get it since he is over 18?

He could start a search for HIS parents.

However, YOUR record is going to be inaccessible to you until you turn 18. Your adoptive parents may be able to help you with it, though, since they are part of that record.