The irony is both were treated the same until the regression. Rosa believes since she was born first that she is deserving of everything [like most kids her age going through the "mine" phase]. She's not at fault for her personality since it's definitely the fault of the mother and the nanny for not instilling proper growth guidance.
I'm leaning towards Joey convincing her to leave to allow better growth on the dark path without having to tread with caution.
I have no sympathy for Rosa; she could have chosen to get along with her sister and grown, but chose the selfish path with a want to kill her sister. Look back at the previous line, she gave her sister a scent pouch with poison in it. She'd been trying to kill her sister even though the girl lived in the shadows being the best support she could for Rosa only to be toyed with from the very beginning.
if I were Rosa I'd want to grind everybody's face in the dirt too, ngl. And lest we forget, these kids are *four years old* LOL. Despite being a princess, Rosa is so isolated and lonely she makes excellent prey for an agent of chaos? If you want your kids to hate each other, do exactly as these parents do, haha. Also, what kind of trustless marriage... this whole family is a mess. Rosa got the right idea to get the hell out XD That red bird is just as manipulative as the black cat, he's just 'cute and sunny' so he skates under the radar.