Uso no Tsukenai Otoshigoro

In Japan, a mysterious and bizarre disease called "dissociative true feelings" has become epidemic among adolescents. The disease is not particularly life-threatening, but when one becomes excited, an alter ego will appear to express one's true feelings without any pretense. Sayoi's twin sister, Satsuki, a second-year high school girl, developed the syndrome after an incident in junior high, which also caused her to lose her voice. Since then, Sayoi has strictly avoided contact with the opposite sex in order to protect her sister, who can no longer speak with anything other than her true feelings. One day, however, she meets a boy, Shougo, who also suffers from the same disease, and the relationship between the sisters begins to change...?

(Source: Comico, translated)

Note: Includes four extra chapters.