Well, she is already a bratty girl with high pride since before she die, but previously she was still a child loved by her parent and she love her parent too. Even though she was being compared to her sibling, and she was studying hard, she still can act bratty, and selfish because she deep down she know she is loved by her parent and she is a child to them, so it is okay to be act like that.
Well human is complicated...
in a group of friend, you might’ve taken the role of the mom being mature and such.
But in your own house, or in front of someone you depend on, you would act all spoiled and childish.
I think the story and plot are interesting but the main character is a bit...bratty and immature for my tastes. She's the type that uses high risk-high reward logic and puts her ambitions first before others, a selfish mindset.