It's when you have no rights to throw away any tantrum you've held so long, while the other one -the older one- has full rights to do so, even rub it in your face. Something about being sibling is the presence of hierarchy.
Being a younger one means you have no rights to tell your elders about what to do and to avoid. Being the youngest has it's own advantages, indeed. Saved by parents. But being the middle is out of the question. Middle is a multiplayer; has double roles. Sometimes you are considered mature enough to act like adult and be a good model for younger. But in your elders' eyes, you might be just a bug under their shoes. You are as childish as another younger. As naive as it could. Fool.
You could have that pride of an older sibling who have to be better than the younger in any aspects; but still, you are not allowed to surpass your elders. Just like them had the same pride. They could be that manipulative.
One is born lucky, the other is lucky to be born. Guess who is who now?