Ichinen Buri No The Animation(2017)

As the summer break begins, a certain college student decides to pay a visit to his family in the countryside. Having been away for more than a year, he wants to cherish every moment of tranquility in sleepy rural surroundings, far from the hustle and bustle of a major city. Greeting him on his arrival is his little cousin, Anna, who refers to him as "Kou-chan." They soon start hanging out, reminiscing their childhood together. Eventually, the two visit an inconspicuous hut—the place where Kou-chan seduced Anna during his previous visit. It is here that Anna is determined to make the most of her cousin's stay and show him how much she's matured over the last year. As memories of their last encounter resurface, she invites him to take the plunge into a whirlpool of sexual pleasure and continue where they started. (This OVA is based on the adult manga by Ayumu Shouji.)