DING! DING! DING!
My older sister could take a butt-whooping like it was nothing! My younger sister (who is older than me, but younger than the rest of my siblings) could not handle the slightest whooping.
So, when they were tasked to do things together, like clean the kitchen, clean their room, wash/fold clothes, my older sister would sit on her butt and do absolutely NOTHING...all the while reminding my younger sister that when it is done, they are BOTH getting a whooping, and my older sister doesn’t mind it! Needless to say, my younger sister did most of the work on most days!! LOL!
That’s great that you could figure out your sisters’ dynamic in early life. Yep; exploitation of others is part of human nature. And why we need to lift ourselves up out of the sins of the flesh as much as possible.
I’ll just say this: parents should nip this exploitation stuff in the bud, and also not play favorites. As you point out, all kids are born distinctly individual. Parents can’t just learn how to be a parent on the first one and apply the same skills to the next—the other kid(s) may not be like the oldest kid or the favorite kid. Each child needs its own well-thought-out parenting.