Excuse me, but as a middle child, I have to absolutely object to that last statement. Middle children have their own unique sibling/personality complexes. :P
Oh wait, I don't exist. Wow, that's a stealthy, mean joke. That cuts, fam.
I think it was less a joke about how middle children don't exist and more a joke about how middle children are often forgotten about compared to the oldest/youngest
Don't exist until they're gone ... I'm a middle child in a family of eight ... mom kept an eye on the baby and the oldest ... as long as there was six in the middle, no one really cared who they were until that number of middle children decreased.