I think the "middle child" (second oldest) comes form the fact that he should be ready to take over the duties of the oldest if he dies but must not grow to be

I think the "middle child" (second oldest) comes form the fact that he should be ready to take over the duties of the oldest if he dies but must not grow to be "better" then the oldest. Can't leave to make it on his own like the younger siblings but by default don't inherit the family fortunes/business/whatever. But as in the west we don't live in a society where the oldest inherits his fathers job/title(s) this doesn't really apply anymore.