:(

I don’t think he did anything to her. More like she was always in his shadow and only existed to make him shine, which made her feel empty or ‘puppet-like’. MC was afraid or going back to that life, which led to the accident that killed her.
¯_(ツ)_/¯

It's portrayed very vaguely, so it's really up to interpretation.
Some say nothing really happened and she just felt overshadowed and manipulated by the brother. Others believe he abused her on top of the manipulation.
But frankly the author never explains explicitly.
I'm of the belief that her past with the brother is of a more severe nature, due to her actions and reactions painting a picture of a severely traumatised young woman...
But I also believe that the author is very deliberate with leaving the past up to interpretation, so that a younger audience aren't shocked by the story, but an older audience see the darker themes of the story.

thank you this saved me in drowning myself with questions and confusing myself even more. the parts where the words were scribbled out and the scenes of some parts made me think more about the dark stuff that might have happened and it is reli unsettling to watch