You're not wrong. In fact, everything is way too convenient for her. She was way too prepared for this. She has precognitive vision levels of preparation and planning. I suspect that she either has the ability to see the future or was involved in her family's death.
oh damn! I hadn't thought that deeply on it but that would certainly be a dark turn that would make sense
Wait, what if it's both?! What if she has the ability to see different timelines and she manipulating events to bring about the timeline she wants. That would explain how no matter what the MC does, the sister is always one step ahead of him.
she was the End Boss all along! xD
uh... you'd think that she'd lead with "all 3 of our older bros are dead!" with a bit more emotion than one might have when reporting quarterly earnings. rather than "you're the head now. oh btw, our family is dead, so things will get busy for you."