Umm I’ probably gonna get hanged for this idea but... When people say love is love why does this not include siblings? As long as there are no children( since the child may not be healthy) I think finding this wrong is not that different from finding gay people wrong 50 years ago? Isn’t this just a person loving one another because feelings are something we can’t control?
I agree that it being illegal is stupid irl since they are two consenting adults but it's still fundamentally fucked up. Also "love is never wrong" is simply wrong. There are so many wrong and twisted forms of love. What matters is what happens between two consenting adults is nobody else's business but theirs.
Historically, some royal families would marry with people who were blood related family with no issue in order to preserve the """"pureness"""" of their royal blood. However, since that means that any genetical mutation would remain and get passed to their descendant, every generation would have more health issues than the last one (see Charles II, king of Spain as an example of this). The reason why it became banned was because every single time there was a royal who married their immediate sibling(s) there would be war. Either because there was infertility because of how fvcked up their genetic pool was or because people wanted power.
Furthermore, there were cultures that didn't have incestual marriages (from the beginning or because they latter banned it because health issues) but that's because they didn't want to maintain the power but also gain more power through alliances while seeking alliances there is nothing more binding than a marriage contract (until divorce appeared and attacked). So, if you have a descendant who will be inheriting your crown and a another one who can marry into another family and will help your heir, and there is no possibility of a fvcked up gen pool... It's a good situation for you and your heir.
Then, throw around some religious sin in an ignorant society and tell 'em that mutations are bad and wicked and the devil's work and you have people reacting veeeery bad to incest.
After this kind of historical lesson on incest, if we were to accept incest between siblings, we would have to accept incest between parents and their childrens, grandparents and grandchildren, uncles/aunts and nieces/nephews, etc. There could be a lot of grooming going around. So nope, incest is bad.
I feel like it’s a very thin line they are walking on