Surprise Attack! Little Sister Warning!! (急襲!妹注意報!!Kyūshū! Imōto Chūihō!!) is the 9th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
A girl named Yuzurina Houshou moves next door to Shougo Mikadono and introduces herself as his little sister. After Yuzurina explains to him the situation, Shogo receives a call from his mother, Kanoko Mikadono, and tells him on bringing with him a female accomplice during the Mikadono Group party. While frolicking in the swimming pool, the girls close to Shogo learn of Yuzurina's sibling relationship with him, much to their disbelief. After Yuzurina agrees to take a DNA test, the other girls argue on who will Shogo choose as his accomplice to the party. Later at his apartment, Konoe makes dinner for Shogo and later presents herself as his "dessert". While Shogo teases Konoe, a bewildered Yuzurina overhears them. Shogo then sees Konoe off to her place, and when she tells him to kiss her, a shocked Miyabi witnesses this. Afterwards, Shogo receives the result from the DNA test, confirming his sibling relationship with Yuzurina.