Responses
It's actually kind of normal for this kind of families to keep the bloodline by marrying like this.
It was common in the past through all the world too, mostly in royalty but in superstitious families too where some kind of supernatural power was hold.
I'm not quite sure if now a days they are families keeping up with this tradition in the more rurals parts of Japan, but it totally could be.
Also the incest taboo between cousins is more a western (and new) custom.

Okay just read the 1st chapter aaaaand they’re 1st cousins..
:(