You failed to understand her maturity at her age.
Not only was she abused, she was kept quite closed from public, which made her lack common sense..and several other knowledge. As well as the idea of Sacrifice, or the reasoning of why one would torment their own child.
Surely you, and other readers, might be able to come up with idea of sacrifice once subjected to it.
But that's most likely because we manga readers have already been exposed to many story settings that we can recognize it quickly.
Because there's a difference between realizing something and having it confirmed so bluntly. Until it came a straight from the horses mouth. Until than she could deny it a bit. Pretend that her family at least loved her a little bit. Also while she knew she was a sacrifice, she didn't know she was born SOLELY to be a sacrifice.
as someone who grew up in an abusive home. it doesn't occur to you that you are unloved. you think that everything is your fault and if you are better finally will they treat you well.
That is how A CONFIRMATION felt like. In her situation (the abuses) it is pretty normal that such VERBAL CONFIRMATION felt like her final verdict. And yes, she finally decides to cut all ties from that family.
i dont think you understand the mindset of an abused child.
I think it can be hard for some people to understand, especially if it's an experience they haven't endured themselves, but there's a large difference between thinking that you're unloved and disliked for perceivably no reason vs having it confirmed by the people themselves that you're only purpose in being born was to be used as a sacrifice for another (you're own sister, no less).
There's even a large difference between thinking you're unloved, and being told you're unloved. At least if you only think it, then you can convince yourself from time to time that it's all in your head. That you're exaggerating the abuse towards you. You convince yourself that you're the issue. You're the one in the wrong. And the abuse isn't "abuse", but "punishment". Something you deserve, even if you don't understand why. You might feel anger, sadness, or any manner of negative emotions towards their treatment of you, but because you think it's somehow your fault, you convince yourself that those feelings are unjustified. You're ungrateful, and rude, a bad person who only further deserves the "punishment".
But, when it's finally confirmed by the people themselves that they simply dislike you because you exist---something you never asked for and have no control over, it becomes more absolute. That you cannot do anything to make them care for you, because you aren't doing anything wrong. You simply exist. For some people, especially over time and with hopefully some therapy, you're feelings suddenly feel valid. You're ALLOWED to be hurt and unhappy, and to feel angry and hateful towards your abusers. What they did was abuse and was wrong---not punishment.
So in chapter 1 her family ties her up and sends her into the burning flames as a sacrifice - they even say shes a sacrifice for her sister, and yet chapter 2/3 when she wakes up the day after the sacrifice she over hears her family say that the only purpose she serves is to be a sacrifice for her sister, and the MC is all like "!!! What?! They only wanted me for a sacrifice!?". It's like.... ? Yeah, no shit, what part of them tying you up and throwing you into a burning fire did you not understand last night?
Like... She had already lamented how badly they treat her, and that shes only there to serve her sister, and shes already been tied up and thrown into a fire, and none of that tipped her off that her family didn't like her. It was only when she heard them say that she's unloved and only purpose in life is to be a sacrifice that she actually freaks out and comprehend that. Like??? How many clues can you miss?? What part of being thrown in the fire did you not understand?? It wasn't until you verbally heard it did you understand that you were used for a sacrifice.
Good story, but that one detail - her being oblivious to the obvious until shes VERBALLY TOLD - that just ticks me off. What didn't she understand? She acted like it was such a shock to be told that she was used for a sacrifice, even when just the night before she was obviously used as a sacrifice!