
I have 4 siblings i interact with pretty similarly
We hate each other but also not really. With people outside our fam we act normally, but among ourselves, anything we're better at than the other or win at, we rub it in e/o's face (not as much after we became adults but yk). It's like an eternal game.
In the end it's just how we work lolol

Yeah that's a must to rub it in your siblings faces, but she's genuinely being an asshole. Especially because being a sibling means you want to be better, I get that, however, what I don't get is how she decides to tell others private info that he didn't share. I would NEVER do that to my brother, because one, it's none of their business, and two, IT'S NONE OF THEIR BUISNESS!!
She's pissing me off. First of all, it is NONE of the school's business in what happens at home unless he chooses to tell them. Just because he's a star, doesn't give her the right to tell his shit to others. Second of all, what kind of fuckass cousin is that.. I'd understand if it was all jokes, and she's joking around, but she's genuinely acting like she's above him just cause of his grades. I hate to break it to her but being smart isn't everything, yeah it's a big part of everything, but it's not all you need to survive, she needs to lower her ego.