Probably because the ML has no background and nobody would care if he suddenly vanished one day. Unlike what will happen if her brother disappears.
i would say yes, that's a bit too game of thrones here. Also, while she doesn't seem to have much of a good vibe going with her brother, I don't think she hates him enough to kill him like that.
Also she needed a specific sacrifice with similar magic wavelengh, ML has magic power so similar he could enter the green house that only reacts to mc magical power that is how much they sync
Ok, I swear I'm not evil or even recommending this, but I had a thought, wouldn't her full blood brother be a better sacrifice? And wouldn't it be satisfying revenge to kill him in front of her mother? So she lost the child she abandoned her daughter for. Or is that too game of thrones for this genre?