beta
Honestly, the reason why the sis is so fucked up is probably because of the expectations that the village society had for them. MC who had memories of a different world knows that incest is weird, but the sis has no memories or knows any ethics behind why it is wrong. Besides the incest part, Gizel was practically groomed and conditioned by the mentality of the village to marry her brother and also, in a way, groomed to have feeling of love and affection towards him. So when MC says no to that sort of thing I can image why there was a backlash. Imagine being raised with people that you'd get to marry your brother and have a fufilling life together, practically a birthright, just to hear that he doesn't like you that way.
Towards another extent it's partially the brother's fault BUT NOT BY A LONG SHOT in comparison to how fucked the village was for pushing them into marriage in the first place not even asking if it was what either of them wanted. But here me out, imagine the boy who you were suppose to marry, who you love, just up and left and never gave you any reason as to why he left. You'd be reasonably either angry or in self doubt. If there were proper communication, maybe she wouldn't be as angry but honestly also doubt that because again GROOMING.
Her actions are inexcusable tho. I'm not really defending her for all the shit she is doing honestly and actually going so far to try and control the MC's life and also harming his companion and teaming up with the DEVIL. However, she is a much nuanced character outside of her actions. But again knowing the reason why she is so heinous isn't an excuse whatsoever especially when she is harming others. She should be held accountable for her actions 100% and I hope this manga doesn't excuse her for all the bad she has done.
Anyways that's my rant