The main reason why incest can be harmful even in the most perfect hypothetical society in which it is socially acceptable, is that you cant just "break up" with your sibling. It can cause a lifelong rift, making it hard to go back to being normal siblings. Breakups destroy established relationships, and destroying the relationships between the smallest social unit (the family) can cause ripple effects on society as a whole.
This scenario is highlighted in this chapter. The relationship between the siblings are now getting awkward. In real life, siblings are very tsundere and prideful towards each other. And its often hard to own up to ones own mistakes. If its hard to say you're wrong in normal relationships, imagine romantic relationships between siblings.