Yoimachi-hime: Taishō Inwai Kyūketsuki-tan

Yoimachi-hime: Taishō Inwai Kyūketsuki-tan is a Japanese erotic vampire visual novel set in the Taishō period of Japanese history.

The protagonist is Tokio Izumi, the heir to an aristocratic family. With his mother dead and his father usually travelling around the world, Tokio spends most of his days in the company of his adoptive sister, Yui. One day, Tokio and Yui are exploring the mansion when they open a mysterious chest, accidentally releasing an ancient horror which turns Yui into a vampire. When she bites Tokio's neck to drink his blood, he is shocked by how pleasurable the sensation is, and his relationship with his sister is transformed forever.