it depends, this may be survivorship bias, because when a brother is a villain, it is usually the eldest
however, when a brother isn’t the villain, the protagonist is usually the firstborn. especially in the Fairy Books, there’s a bunch of mentions of “so-and-so had other 19 younger brothers and sisters” and things like that
yeah, I usually think of books the same way as someone telling me a story
like, if my mom goes “and then I went to the bank and saw Judy there” I don’t picture the bank and Judy, I just kinda acknowledge the existence of both in my head, idk how to explain it