Tatsuya and Miyuki are biological siblings (they have the same parents). Miyuki was genetically engineered and because of this they are far genetically. These details are reveled in volume 16 that is being adapted to manga, the Yotsuba Sucession Chapter has 09 chapters translated to Spanish here.
Tatsuya and Miyuki can get married because their aunt altered Tatsuya's registration in order to engage them and their far genetic relation permits them to have children.
TLDR; They are biological siblings who genetically could test as cousins and are legally first cousins due to the documents alteration/Maya's official statement to the their community.