It’s a morals thing. It’s immoral. They were raised together as a family for 8 years during their prime developmental ages. If they weren’t raised together for that long then maybe some wouldn't see it as immoral, but it’s hard to set aside that fact since it was that long. I doubt at their young ages they were thinking “we’re not related by blood” or distinguishing their relationship as stepsisters. Also I doubt they stopped their relationship cold turkey once their parents divorced. So to most people they were fully sisters despite them sharing no blood. Still read it cause it’s fictional, but I do see it as a little immoral and weird.
Also one more thing, it’s not about being wrong or right, it’s about it aligning with your morals or not and everyone has different morals and beliefs.
okay so i may understand why this makes me people uncomfortable but the POINT is that their parents who were remarried DIVORCED. you could say they were sisters, but not by blood and since their parents are divorced, by law they are no longer sisters. therefore, it’s not wrong if they are in a relationship now although the sentiment of them being sisters still remain.