I get where you're coming from, but in the end what you're doing is showing sympathy for the perpetrator. Which in the end results in harm towards the victim.
Yes the village was wrong in grooming her too an extent. They have a backward of thinking that wasn't that uncommon in medieval times. But the sister is taking it multiple steps too far, to the point where nothing but her herself can be blamed for what she's doing.
There's "being obsessed with the person one was groomed to be with" and then there's "willingly selling one's soul to the literal good who owns too destroy the world." The latter being several magnitudes worse.
I don't even blame MC. Sure he didn't ask about his marriage, but would you go up to someone and ask "Hey, am I expected to marry my sister?" He also showed in his actions that he wasn't interested in marriage (the sheer fact he didn't bring it up unless someone brought it up first being one of them). I blame the father quite a bit though. He was like "MC gets roped into things easily, so I'll just force him to marry his sister not even a few days after MC found out he was expected to marry his sister. He will get over it eventually." Yeah MC left, but he was actively put into a position where he didn't have another choice. The people around him made it clear that they didn't value his opinion. His family didn't nor did the rest of the village.
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