@GrayBypasser I said same-sex incest is better than opposite-sex incest, but that doesn't change the fact it disrupts the familial hierarchy. And it's not just same-sex incest. There doesn't need to be a blood relation, to fall in love with anyone you grew up with/who raised you is kind of really weird. But I'm still figuring out my stance towards these kind of complicated "family" relationships. But to sum up, it's not about offsprings, I just can't imagine falling in love with my mother, brother or anyone like this. Even if they told me I was adopted, there is no way I would develop romantic feelings towards them. I love them, but it's a purely familial love. But it's not like I know where this webtoon is heading. If they can properly portray and develop the relationship of sisters, then I won't really have a problem with it. So far I didn't see any romantic hint, but considering the pace of Ouji-sama it's still far off. However, I still usually accept incest only in twisted stories where they are not romanicized.
PS: Just saw that someone replied to you already. Btw I don't think it's wrong to fall in love with your sibling if you don't know you are related. That's normal. Just don't have kids when you find out about blood relation. But it's a really complicated topic. I just think it's wrong to suddenly perceive your sibling as romantic interest.