Hatsumi Shiina (main girl) = wife of Kouji Kouji Shiina = husband of Hatsumi Takeshi Shiina = older brother of Kouji and Hatsumi's older brother-in-law Shuuhei Shiina = dad of Kouji and Takeshi and Hatsumi's father-in-law
So basically Hatsumi cheats on his husband, Kouji with his brother Takeshi. Classic NTR.