There seems to be a minor translation error on the bottom of page 12. Tsumugi would not be Asahi’s “brother in law” because that would mean he’s Nana-san’s brother, not Saki-san’s child—your brother in law is the brother of your spouse. The correct term to describe Asahi and Tsumugi’s relationship is half-siblings, as they share the same father but have different mothers.