Neither of the above but knew of many people, including my mother an aunt, & a first cousin once removed who were oldest/older sibling. The oldest/older sibling is the one that the younger/youngest siblings look up to for advice & guidance. The oldest/older sibling is the one who is the example setter for younger/youngest siblings to emulate. If the oldest/older sibling errs, the younger siblings learn from the mistake & most likely aren't going to do what the oldest/older sibling does.
Oldest/older siblings teach younger/youngest things that parents can't, don't, or won't teach. They will also tell the latter things that parents won't tell. There is more of a generational bound between oldest/older & younger/youngest siblings than it is between parent & child.
Oldest/older siblings have weathered difficulties so that younger/youngest siblings will have a much easier time. They are there when the youngest/younger siblings need them. They are the strong ones that youngest/younger siblings go to. They are viewed by younger/youngest siblings is the strength of the family- the fixer upper & the responsible one.
Oldest/older siblings are seen as being powerful, even authoritative by younger/youngest siblings who admire the former's leadership ability. Younger/youngest children feel powerless so they want the power that oldest/older siblings have. Oldest/older siblings have the status that younger/youngest siblings want.....even crave.
Oldest/older siblings are adults in the eyes of younger/youngest children. To younger/youngest siblings, oldest/older siblings can do things that they aren't allowed to. Oldest/older siblings can date, stay out late, & do other adult things that younger/youngest siblings cannot &/or aren't permitted to do.
Oldest/older siblings also bear the brunt of the familial responsibilities so the younger/youngest siblings won't have to. They also bear the brunt of the discipline & punishments for what the younger/youngest siblings do with the note that the former should know better since they are the older ones.