I'm not sure if cousin marriage was or is a common thing in Japan, or atleast not as frowned upon as in the west. Cousin marriage used to be a common thing in certain European monarchies, and still is a common thing in a fair amount of countries today (e.g Pakistan, Egypt, Mauritania).
Since Ayn and Thea are second cousins, I think it's passable, especially since most towns likely don't have huge populations.