Saikin, Imouto no Yousu ga Chotto Okashiinda ga.

Synopsis

With the two locked in the bathroom, Yūya gives Mitsuki his shirt and the two share bits of their past with each other. The result of the talking causes a reaction in Mitsuki that fills up the heart a little. Yūya calls out for help and is answered by Yukina who comes to let them out. Yukina introduces herself to Mitsuki as Yūya's childhood friend causing her to become upset while thinking about it. Later on, Hiyori furious that Yukina is making moves on Yūya warns Mitsuki about it telling her that relationship stress and jealously make the heart's contents on the belt drop faster. Hiyori possesses her again and flirts with Yūya while trying to humiliate Yukina, the result of which has little effect. Determined, she gets Yūya to admit that he likes both large and small breasts to Misuki's embarrassment. The result also causes the heart on the chastity belt to recover and fill more. The next day, an accidental spill causes Yūya to see Misuki's panties. After putting her panties in the wash, she refuses to walk to school with Yūya and Yukina. Hiyori suggests that it is jealousy, something Mitsuki blushes to deny. Hiyori also notices that Yukina is looking directly at her something that she dismisses as not possible. While at school Yūya is embarrassed when thinking of Mitsuki's panties he had accidentally seen earlier. Meanwhile, Mitsuki acknowledges that she forgot to put her panties back on before she came to school after washing them. During lunch, she is invited by Ayaka to join her but declines noticing how windy it is. Yukina hints to her that she knows her problem and helps her out by trying to bring her back inside. Things go okay until a gust of wind exposes her butt to Yūya causing him to fall back out of shock and hit his head. The episode ends with Mitsuki finding a new pair of panties on her desk that she receives from an unknown person. Yūya meanwhile is in the nurses office with a headache and memory loss of what happened with Mitsuki.

(Source: Wikipedia)