My Sister Was Always Beside Me (妹はずっと、そばにいたImōto wa Zutto, Soba ni Ita) is the 12th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shougo Mikadono's and Miyabi Kannagi's date ended with them going to the Lyrical Sister Café, after which they went back to Shougo's apartment complex. Miyabi suddently gave Shougo a kiss and asked him to be her boyfriend, but she was turned down. Miyabi tearfully stormed off. The entire event was witnessed by Yuzurina Houshou.
Shougo, with the help from Ikusu Mizutani, revealed to Yuzurina that he had discovered that she was not not his real sister, but a child actress named Nayuri Danno who played "Perin" in "Transforming Warrior Granberion". Yuzurina had been working in secret with Risa Seri, who altered the DNA test results, and the Mikadono Group's business rivals to create a scandal between him and Miyabi, his real sister. Yuzurina admitted the truth and left.
Shougo received words from Mei Sagara that Miyabi almost drowned during her pool practice and was sent to the school's infirmary. Shougo correctly called out Mei to be Miyabi's childhood friend before went inside the infirmary to meet Miyabi. She confessed to be the one asking Shougo to marry her back at the funeral. Miyabi discovered her parentage after Kumagorou Mikadono sent her a letter alongside the money, but decided to hide her connection to Shougo to allow them to be lovers.
When Miyabi confronted Yuzurina about her deceit, the actress instead demanded Miyabi to prove her relationship to Shougo. They had agreed that Miyabi would give Yuzurina Kumagorou's letter at the party, but at the last moment Miyabi didn't do that and instead allowed Yuzurina to continue the charade, while Miyabi would become Shougo's lover. Shougo forgave Miyabi and reassured her they would always remain as siblings.
Some time later, Shuogo, Mei, Konoe Tsuruma, Rinka Kunitachi, Mana Tendou, and Maiko Kotori came to a swimming competition to cheer for Miyabi. Though Miyabi did not win, Shougo congratulated her for having broken her personal record.
Ikusu reported to Shougo about her failure to locate Yuzurina, though she had taken some steps to prevent his enemies to make a move soon. Shougo was later chased around by the girls after they had heard of the rumours surrounding him and Ikusu.