Koibito wa Yawachichi Onee-san Hoken'i ~Tomecha Dame yo? Watashi ga Suki nara Ninshin Sasete♪~

The protagonist, Souichi, is a plain and sickly school boy.

Chitose is the nurse at his school's infirmary. She's also an old acquaintance of his, and Souichi is in love with her.

One day, both of his his parents are suddenly appointed to work overseas. Chitose agrees to become his guardian, and they start living together!

The two of them become lovers, and a new life filled with sexual indulgence and child making begins.

[From English DLSite]