Yosuga no Sora

Season 1, Episode 2: Akira Embarassed

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Sora asks Haru to take her measurements so she can have a uniform made, and finally starts attending school with Haruka, though she is disappointed to find that she and Haruka are in different classes. While cleaning the swimming pool as part of a mixed gym, Akira accidentally soaks Kazuha who forgot her swimsuit, and so Akira takes her to dry off. Haruka finds Kazuha's phone and goes to return it, only to overhear a sensual-sounding conversation in which Kazuha addresses Akira as "Onee-chan" (big sister).