The story is never about the middle kid. The eldest gets to be the heir, or sometimes the lost heir, or occasionally the bully or the man in charge or the villainous oppressor (or the mimetic-realism equivalents of the above.) The youngest is the bold one who goes out into the world to find his fortune, or the virtuous and kindly one who wins while the older siblings are undone by their own unpleasantness, or sometimes the mysteriously adopted foundling or the one with special powers.
But nobody ever tells a story about the middle kid.
This is a law of storytelling that could, under the right circumstances, be profitably broken. But it would take work. A novel of mimetic realism would want to make itself into a family problem novel about the angst and trials of being a middle kid; a fantasy novel would want to deliberately subvert the archetype. Novels in other genres would want to handle the problem by thrusting the protagonist’s family so far into the background that he might as well have sprung fully-formed from the brow of the CIA, or the USMC, or the NYPD, or the faculty of Harvard Law.
The last-named case moves us over into “action heroes don’t have families” territory — which is a profoundly unrealistic motif. But nobody wants to think about the noir-story LA private eye with his trench coat and his fedora and his world-weary outlook going back home to Toledo, Ohio, for Thanksgiving, where he spends a long weekend not as the owner and sole operative of AAA Investigations Incorporated, but as Joey the middle kid who gets ribbed by his siblings for showing up looking like a poser on TV and whose mother wants to know if he’s met any nice girls yet out there in California. After four days of this, murder and palm trees and brutal cops and corrupt city officials start looking really good.
Writing an ordinary family with no more than the normal amount of inter-sibling and parent-child conflict can be hard work. It’s no wonder, really, that so many writers resort to making their protagonists orphans, or to giving them dysfunctional families that they don’t talk to any more. But it does leave a lot of open territory out there, just waiting to be explored.