My Sister Resisted (妹はあらがっていた Imōto wa Aragatteita) is the 8th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shogo gets himself admonished by Rinka for not properly acting as a morality officer. Afterwards, Shogo and Ikusu have a double-date with Rinka and Mana at an amusement park for the purpose of thoroughly investigating Rinka as suggested by Ikusu. After their date, Shogo takes notice of a phone accessory he bought from the park, resembling one that Mei had, which Rinka once noted she had seen it before. Shogo confronts Rinka, concluding her to be the one who blackmailed him. Although admitting this, she solely wanted to warn him that Genda, one of the board members whose son is betrothed to Rinka, tasked her to investigate information on the illegitimate daughter to ultimately blackmail the Mikadono Group. Against with the scheme and the engagement, Rinka used Mei's phone and its voice-changing function as a means to express her contradiction. Reflecting on the time they spent during their date, Shogo volunteers to become Rinka's fake boyfriend to call off her engagement, which turn out to be successful. Afterwards, Rinka pursues Shogo to become her true boyfriend.