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. She explained that until now, Yuzurina had been under the guise of Shougo's second cousin on Kumagorou Mikadono's side. She would affraid that Kanoko Mikadono would not take the news well if the truth were to be revealed.
Shogo right afterward received a call from his mother, who told him to bring along a female companion to a party hosted by the Mikadono Group.
The day after, Yuzurina revealed her relationship with Shougo, much to everyone's disbelief. As all of the documents proving she as Shougo's were destroyed, Yuzurina agreed to take a DNA test. Risa Seri, who also knew of Yuzurina, was entrusted by Shougo to arrange for the DNA test in secret.
Then, the other girls began arguing on who would accompanied Shougo to the party. That night at his apartment, Konoe made dinner for Shogo and then "presented" herself as his "dessert". While Shogo teased Konoe, a bewildered Yuzurina overheardthem.
Shogo saw Konoe off to her place, and when she told him to kiss her, they were witnessed a by shocked Miyabi who ran away. Just having returned to his apartment, Shougo received the positive DNA test result, confirming his sibling relationship with Yuzurina.