
This is Japan. Even if they are really cousins, it is allowed. Half of the earth allows first-cousins marriage. In Asia I know only 2 countries so far - Japan and China. Plus one of their parents could be the first cousins, as seems they don't share same grandparents, so that makes them second cousins and majority of the countries allowed it.

Half of states (25 states) is small number compare to half of earth lol. Not saying first-cousins marriage is right or wrong, I came from an Asian country who doesn't allow even allow second-cousins marriage, even so first-cousins. And I am not a fan of incest too - whether it is legal or not. I am just stating a fact about first-cousins marriage in Japan (reason why cousins romance is a normal trope either in romance or even hentai manga) and why I won't pretend they are not relatives :)
Before I read this are they really cousins?