I see it mainly in Fantasy Books.
One Brother is the good one, he is manly, strong, don’t care about riches, and have a good heart. He will be the protagonist or the protagonist friend.
The other one is the Bad Brother, he have a more weak body, have femine traits, always using fancy clothes and jewels, is less loved by others, and he is jealous. He will be the antagonist.
In a arc of >!Ranger!< this trope is reverse, which was a breath of air.
Also, in fairy tales, the oldest child is inevitably the villain. There might’ve been some truth to that back when the eldest (usually a son) inherited the family’s wealth, but it’s no longer universally applicable, especially in America where younger children tend to be favored.
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