Saikin, Imouto no Yousu ga Chotto Okashiinda ga.

Synopsis

While lying in bed thinking of Yūya Hiyori tells Mitsuki to respond more to him and says that she could be the ideal little sister, something she takes as a challenge. The next day, Hiyori possesses her and makes breakfast for Yūya who is surprised. The real Mitsuki watches all of this and dismisses it as "sucking up", and has Hiyori reassures her that she won't do excessive body contact or seduction on him. She continues to watch as Hiyori acts very sister-like to Yūya while in her body to the point of causing him to smile. Realizing that he had never smiled to her before the real Mitsuki begins to go deep into thought. Her thoughts are interrupted by Yukina jokingly saying that it looks like the two are on a date. Neko walks by where the real Mitsuki is floating saying "you shouldn't stay separated for too long" causing confusion if she can see her. Later on Hiyori who is still possessing Mitsuki goes to a drainage ditch with Yūya to skip rocks. The real Mitsuki floats nearby thinking of what being real siblings means. While skipping rocks, Yūya talks about his past and of a girl he had as a friend who called him "big brother". Upon hearing this, Hiyori as Mitsuki jumps into his arms saying it was her and happy that he remembered. When the real Hiyori tries to intervene she discovers she can't go back into her own body. Later that night when she tells her she can't go back into her body it makes Hiyori want to take advantage of the situation. Hiyori as Mitsuki insists she sleep in Yūya's room with him because she is scared of the thunder and later crawls into bed with him. Hiyori then says through Mitsuki that she will be the perfect little sister to him, Yūya responds by saying that she should just be herself. The result of him saying that, causes the real Mitsuki who is floating nearby to feel touched and Hiyori to leave her body. Hiyori then literally kicks Mitsuki back into her body, in embarrassment of everything that happened Mitsuki then kicks Yūya out of his own room. The next day Mitsuki tells Yūya she is going to be as she has always been like he said.

(Source: Wikipedia)