18+ Tsun na Kanojo Dere na Kanojo

Hiroki has an old friend, Misato. She always takes a good care of him. He also has a younger sister, Tsugumi. She is a little bit childish, but cute. He lives a peaceful life with them. But one day, his school merges with a girls' school nearby. There is some confusion among students, but they are back to peaceful life soon. However, a big change happens just before summer holiday. A girl suddenly comes to his house and says, "I'm your fiancee...." Triggered by this, his friends who secretly love him change as well....

(Source: Hau~ Omochikaeri!)