"No marriage may be effected between lineal relatives by blood, nor between collateral relatives by blood up to the third degree of relationship; however, this shall not apply between an adopted child and any of the collateral relatives by blood on the side of the adoptive relatives."
— Article 734, Civil Code of Japan. The prime source of this phenomenon.
Jack: What's coming?
Locke: Something mysterious, but first here comes a father and a son. And a Korean who speaks perfect English, but nobody knows it yet. Oh yeah, there's also a hot blonde who's had sex with her brother, but they are not blood related so it's okay... kind of.