18+ Issho ni Ecchi

After answering the summons of Momoka, the school nurse, the protagonist is forced to engage in various sexual acts with her.

Later, the protagonist accidentally witnesses Momoka's younger sister, Maron, masturbating. He is discovered and Maron counterattacks by stripping him and giving hand and foot jobs. One thing leads to another and he ends up taking her virginity.

Finally, the three of them go to a public bathhouse and end up in a threesome play.