After school, Akira Sakagami is about to ask his classmate, Nana Shirayuki out on a date. It was good until she called out to him, but he was interrupted by the appearance of his sister Rio. 
For Akira, Rio is not only his sister but also his first love. Rio used to live alone, but due to a housing situation, she has now returned to her parents house with Akira. When Akira is told by Rio that Shirayuki looks like her, he sees Shirayuki and Rio in her. 
That night, Akira is seen masturbating by Rio. While Akira is frozen in place, Rio is unfazed and makes fun of the actress he was using as a joke, saying she looks like her. Not only that, but she also grabbed Akira's thing.

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His older sister Rio had moved out of their family home, and it seems she’s about to get married… 
Having seen through her younger brother’s feelings—which weren’t quite brotherly love—Rio reaches for her little brother's crotch…? 
“Do you think it would feel good, if you could stick it inside your big sister’s pussy?” 
Even though they’re siblings, they can’t stop having sex… 
Their private feast of carnal desire knows no bounds!
 
(Source: Frances Shoin, translated)

Rio Sakagami