Follow up on Darketower
I was actually frustrated with the commenters attacking the MC, because I did, in fact, take the MCs side on this. His parents were literally gonna force him into the marriage, with the father letting it off as "he'll get used to it." Reading your comment now makes sense though. I mean. he's lived 16 YEARS. And not ONCE he's been told he would marry his sister? Meanwhile, the mother taught their daughter about this tradition? Definitely off knowing this thought process. It should be noted, he knew exactly what he was doing when he ran away. The fact that he knew he was "breaking a rule" and would be treated as a "criminal" implies he knew SOME bits of Marren Tradition. But Sibling Marriage? NAH. He didn't know about that cuz he was too obsessed with magic or something. He only learned some parts of tradition, but not the full extent? I guess that's possible... but too much of a deus ex machina explanation if you literally don't have any explanation for his "disconnected knowledge." This, along with the fact that he knew about the "doom clan" that Mera is from implies he learned about other races too? So like. To what extent is his knowledge?
Also incest is bad. There is genetic evidence backing this up. I think a lot of people, including the writer's themselves, are forgetting this is an isekai. These "Humans" don't need to be the same "human." The fact that they're able to use magic is a big difference in biology, or world setting. There's no "rule" saying incest is scientifically/biologically wrong when it comes to other world races, even when they look, feel, and do as humans do. Considering the Marren tribes tradition is to marry siblings of close approximate age together and there hasn't been any reported problems means either A, the writer didn't think about it/write it into the story (which is fair). Or B, there are no genetic/scientific consequences to their action.