Watashi Datte Seishun Shitai Desu yo, Hontou wa. - Ch. 1

[TheLostLight]

Why do they have to share any blood relation other then to fuel incest fantasies.

I'm about to blow your mind here: every single human on earth has a blood relation to each other 🤯

First cousins are genetically closer to strangers than they are to being siblings. And as someone already pointed out, first cousins can marry in Japan (which is actually the standard for most of the world. USA is the odd one out here.)