After successfully proposing to his beloved girlfriend, Kurumiya Haruka, the protagonist, Tsukihashi Kazushi, goes with her to visit her family home, where he meets Haruka's younger sister, Yuki.
That night, Kazushi stays over at Haruka's house and falls asleep in the living room. As he sleeps, someone slips under his covers. Assuming it is Haruka, Kazushi sleepily pulls her into his arms, only to discover that the person beside him is none other than Yuki…
The story portrays a forbidden love triangle between Kazushi, Haruka, and his mischievous future sister-in-law, Yuki.
(Source: Japanese = natalie.mu || Translation = MyDramaList)
~~ Adapted from the web manga "Kanojo no Imoto: Han Nenkan, Yuwakudarake no Sankaku Kankei" (彼女の妹～半年間、誘惑だらけのサンカク関係) by Megi (女狐).
