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An OVA based on the hit pornographic-yet-somehow-not-classified-as-hentai manga, revolving around the siblings of the Aoi family.
A boy named Sora lives in an apartment with his older sister Aki and his identical twin sister Nami. Aki is naturally gifted at sports and school, but she's lazy so Sora often has to clean up after her. Meanwhile, Nami is a bit petulant in her own way, forcing Sora to cook dinner and doing her own thing.
One day, Aki invites Sora to take a bath with her after practicing tennis in drag. Once inside, she starts feeling him up and eventually gives him a handjob in the bath, but Sora is embarrassed because...well, they're siblings. This makes things real awkward for the next few days.
While Sora tries to avoid seeing his sister after that, he gets locked in a gym equipment shed near the school by accident. To make matters worse, it starts raining outside. Meanwhile, Nami is spending the night at a friend's house, and when that friend wonders about Sora's condition, Nami kicks her to the floor and rips off her shirt, but the girl can only think about how Nami looks just like her brother.
Eventually, Aki manages to find Sora, the two of them go home, and have less-awkward sex. The end.