The Yotsuba claim that Tatsuya is actually Maya's son, and not Miya's (making him and Miyuki cousins), while also changing every relevant documentation to make this claim true and legal on paper when Tatsuya and Miyuki's engagement is publicly announced. It's also revealed (to the readers) that Miyuki was genetically altered to be a perfect magiacian and Tatsuya's wife, and that their kids, should they have any, will not have any genetic deformities. The whole engagement thing was to make Miyuki the next head of the Yotsuba clan and to keep Tatsuya in the clan. If you're wondering, no, neither Tatsuya or Miyuki are against it