Sugar Sibling

An older sibling who spends money on their younger siblings without making them pay it back.

My sugar sibling just got a new job and makes eighty-grand a year. I'm going to be spending it like it's mine. They're my sugar sibling.

Big-little sibling

A younger sibling who displays characteristics that the oldest should possess.

I'm the youngest of my siblings, but more mature. I'm a big-little sibling.