TL note: On page 1, Masaki uses the term “義理の姉” (giri no ane), which means sister-in-law, and on page 6 Shu uses 義理の兄 (giri no ani), brother-in-law. Instead of putting sister-in-law/brother-in-law down as text, I opted to go with onee-san and onii-san, as you can use these terms to refer to the same meaning.