https://en.wikipedia.org/wiki/Family_law_in_Japan#Marriage > Lineal relatives by blood, collateral relatives within the third degree of kinship by blood #2, may not marry, except between an adopted child and his/her collateral relatives by blood through adoption. #3 (Article 734)
Huh, the more you know. Ain't got nothing on Alabama and the Muslim world's first-cousin marriages though.
What do they do?