My Sister's Seduction (妹たちの誘惑 Imōto-tachi no Yūwaku) is the 3rd episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shougo Mikadono woke up to find Ikusu Mizutani suggested to him that they should marry each other first before Shougo's little sister could do so. Just as Ikusu pushed Shougo to the bed, Konoe Tsuruma and Miyabi Kannagi caught them in their act, which caused the girls to mistake Shougo to be gay. Later that night, Shougo's little sister called again to tease him, only to refer to the incident earlier.
For the next few days, Konoe and Miyabi tried to "show him the appeal of women" by taking care of Shougo like younger sisters. Their approaches haunted Shougo, as he was worried about unable to control his feeling and committing incest without knowing.
Thanks to the advice, Shougo finally decided to approach Miyabi, and she invinted him to accompany her to shop for a new swimsuit. At the shop, Shougo noticed a swimsuit made by Ryouasu, the camera & mobile phone coorporation which had just branched out to sporting clothes. He deducted that there would be a great chance Kyouasu to become the Mikadono's business rival in the future.
It was raining on their way home, and Shougo saved Miyabi from being hit by a truck, though both of their clothes were wet because of the splashing water. Just as Shougo was about to take a bath in Miyabi's dorm room, he noticed an old Ganizeus figure from Gran Belion, his childhood favorite show. Shougo came to the conclusion that Miyabi was his unknown little sister, and then asked her who Miyabi would like to marry. She answered that it had always been him. Shougo refused her confession.