
Ever heard of 'grooming'? Even if the couple are both adults there is just a huge power dynamic between family members that make incest not okay even if they don't have children. A father could raise his daughter in to believing that their relationship is okay. Or an older sibling could put pressure on their younger sibling. And if the relationship is abusive than the victim can't go to their family who are usually the first to help in abusive relationships. It's dangerous.

Imagine being a mom and have your ex husband start a relationship with your daughter. Even if they aren't biological related it is just completely wrong and it will tear apart the family. You are supposed to be able to trust your family and if they lust after you that trust is broken. If a brother confesses to his sister and she rejects him their familial bond gets broken.

Um then I guess all relationships between men and women are banned, boss and underling, noble and commoner (guess that marriage between Harry and Meghan never took place). After all there's a power differential in each and every one of those relationships and thus the potential, AND realization, of grooming. Oops? What about kids who are related and find themselves adopted into different families only to find each other later on? It happens. But no grooming took place, they didn't even know they were related. Plus the fact this happens proves you are not right about the grooming. Ktbn.
Honestly, just skimmed the comments and people are either "waste of my time it's incest" or "is this.... actually, no but actually, incest" I mean.....what is this? Are you not familiar with this branch of entertainment? That is honestly the only thing that makes sense because anime and manga are full to the brim with it. I don't even understand what's so wrong about incest, so long as it doesn't result in kids isn't it pretty much whatever? But I strayed from the topic at hand. I suppose it's just the newly baked ones that are not used to this, because it's a reoccurring theme in all genres. I mean, if you never heard of twincest is wincest, or it's little sibling incest is wincest, then I'm just assuming you're pretty new or on the normie side of shipping.