One Love ~Yoru no Boku wa Ecchi na Onee-chan no Omocha ni Naru~

Yuu has just started his sophomore year but his cousin, Rin, suddenly comes to his school to work as a trainee teacher. Since they haven't seen each other for a long time and Rin has become quite beautiful, he starts to see her as a woman. However, she still treats him like a younger brother.

One day, he accidently picks up Rin's underwear and is discoverd by her. Surprisingly, she tells him that she would help him. And so, his erotic life guided by her begins.

[Translated from Official Site]