My Sister's Hero is Her Big Brother! (妹のヒーローはお兄さま! Imōto no Hīrō wa Onīsama!) is the 4th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
The following day at school, things get awkward between Shougo Mikadono and Miyabi, which brings concern for Konoe. Shogo tells Konoe that Miyabi may be his little sister. Ikusu later contacts Shogo to inform him of a prototype phone, having voice-changing capabilities, which may have connections to the calls he receives from the mysterious girl who claims to be his sister. After realizing that Konoe might have that phone, he searches everywhere for her, finally finding her at a park holding the phone. She reveals that she was the one kissing him in the photo he received, and she explains that they were childhood friends when his father was alive then. She then also says that Shogo injured his head while saving her at a crosswalk from an oncoming car. Konoe confesses that she used the altered voice as a prank, not realizing how much trouble she has caused for Shogo. When Miyabi finds the two, she reveals that she and Shogo used to pretend to get married in the courtyard of the hospital during her visits. Since both Konoe and Miyabi want to become his wife, they decide to start a battle to see who would be the better wife.