The story is set in middle ages like setting. Non-blood-related incest marriage was far more common by then. It's true that they had really good family moments earlier but now that they are adult and that their feelings change, there is not much which can stop them since it happened quite often for nobles in middle ages so the pairing can still go on. Many nobles in that era adopted someone just to then marry them into the family. Different era, different culture, different value. You just can't force modern view onto this.
I myself don't like incest but I can still appreciate the story. I just don't focus on the incest. That's why I'm still reading this manhwa. And this is a manhwa. A story. A fiction. Not an educational book and especially not the bible that the reader is reading to learn and follow. Have those guys who complained never watched or read good stories that involves murder, especially those with "righteous" motives? Does appreciating those stories means that they also condones murder and also murderous in their real life? No, right? Why? Because they are just stories.
Actually, the fault actually lies in those readers who complained. The note in the summary clearly told us that there will be incest in this (there was even an incest tag which was deleted because they said those siblings are not actually blood related), so why are these guys still reading this if they hate incest so much? They are not actually illiterate that they can't read the note (or the tag), aren't they? Did they also went complaining those incest manga the Japanese authors created so often all these years? Like, did they also find obviously incest manga, read the chapters far until several volumes, while keep complaining all along why there is incest in those manga? That is honestly sad. You don't like it, don't read it. Why do you keep torturing yourself and others (that got disturbed by your complains)?