That's been a thing since way back near the start. Because when her cousin lost his dad she cared for him like a real sister he developed feelings for her as a kid. Not sure if Emmi was aware but Remilia noticed way back then.
I'm assuming it's based on real history where marriages between cousins were acceptable and sometimes preferred.
Super gross and awful of him to treat her that way on purpose though. Even worse than if he was affected by the potions. And I'm not excusing the rest of them either. They all deserve to be regretful about it for the rest of their lives.
i feel like thats the most desperate reason they could come up with to put him in the plot and make him a disgusting "love interest"
Also fair. Especially if he wasn't a psycho obsessive character in the original game. I don't think he was, but I honestly can't remember. Definitely very gross though
amazing ass chapter but whats with the random psuedo-incest plug