PURE×LOVER -姉と幼なじみとHなカンケイ-

The protagonist is just a normal high school student living in a small town. However, something finally worth celebrating happens to him just before summer vacation: he had sex for the first time with two girls at the same time. The first girl is his older sister, Hasumi, while the other one is his childhood friend, Shizuno, who is the same age as him.

After receiving confessions from the two almost at the same time, the protagonist wondered for a while what to do -- until, a certain desire awoke within him, and he decided to go out with both of them. That desire is... sexually training the girls!