I kinda hate it that my brain immediately goes to "yeah, I guess that setting kinda makes sense".
The girl loses everything and is being put into a new family, where she meets the boy. The boy is always there for her, trying to make her smile. Shortly after, she falls in love with the boy, but they are also locked in that not-related-by-blood-siblings kind of relationship and he just keeps treating her like a sister. She probably keeps trying to drop hints, which he doesn't pick up because in his eyes she is a sister and nothing more. Along the way, she starts to hate the sibling relationship that is stopping her feeling from being noticed and, in turn, taking it out on a boy who keeps being locked in that relationship (hence all the "having you for a brother is absolute worst" cuts) - she hates "the brother", not the person specifically. Now with the "ugly bitch" hanging around all the time, and them growing up, possibly nearing their graduation, the time is running out and she's at her wit's end and is ready to try anything. It's a tsundere that's actually justified/explained within the setting.
They could have probably skipped the murder-suicide part, felt kinda unnecessary. Otherwise, still unclear what the overall tone will be, but why not.