@onlyonew
Personally I don't feel like Will is a dick in anyway, he was forced into a marriage he didn't want, was ruined, hunted down and eventually killed for it. Reborn he grew up surrounded with stories saying that he deserved his fate and the only other person who he felt would understand...
@ThirtyDigit
Marriage to a 1st cousin in Japan is legal although it's not as common as in the past.
That's why Japanese stories tend to have it, it's just a cultural difference.