Oh yeah I've heard of those too! But an author like Fujitsuka,as that person is now, won't definitely do that and it's actually quite normal in japan if cousins get married,(in ancient japan,incest[even brother-sister] is not a taboo and used mostly by noble households) tho today most people who do this are those in high society who'd like to keep their riches to themselves or bloodline of the clan~ ٩(๑❛ᴗ❛๑)۶

I like the couple but I have trouble getting past them being cousins. I mean what if they want to have children in the future; they'll have physical issues since it's inbreeding. I might be thinking too much. This is fantasy after all. Anything can go and anything can happen I suppose.
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