Question Author
Older sibling has it rough because they get to watch the younger baby get all the fuss and attention and, if they lack any memory of infancy*, probably wonder why they never had any of that.
Younger sibling goes through -nearly- all the same experiences and close parental attention enjoyed by the elder sibling but it's not quite the same because there's an 'audience' looking in on the proceedings and parent's attention is divided.
So I'm wondering if it's always/necessarily the case that the jealousy is always the eldest envying the youngest or it's just that the eldest always has the edge in intellectual development, knowledge, height, weight, strength and thus always wins the fights?
* I speak from personal experience - my earliest memories are from the age of 3.5 to 4.