I strongly suspect the terms arose in the past from legal terminology around inheritance. It was very common in most countries for the eldest son to inherit the

I strongly suspect the terms arose in the past from legal terminology around inheritance. It was very common in most countries for the eldest son to inherit their from their parents, and in families where there were no sons it was often the nearest male relative like a cousin. A cousin on the father's side had precedence over a cousin on the mother's side, but in English there aren't specific terms for those. Perhaps in Chinese cultures in the past it was even more complicated, and necessitated terms for specific older and younger sibling's children depending on whether they were the offspring of a brother or a sister.