WMG / The Wolverine

The real reason Shingen wanted Mariko to marry Noburo Mori.

Most viewers assume the movie versions of Mariko and Kenuichio aren't related because their kinship (they're half-siblings in the comics) isn't stated and they were romantically involved at some point. Kenuichio is Shingen's illegitimate son like in the comics but Shingen managed to keep it a secret and plans to use Noburo as a way to separate his children without having to tell the truth. Shingen also fears any children from a relationship between Mariko and Kenuichio will become mutants. (He describes mutants as "God's mistakes" in the film)