From JShoujo Scans:

Chie's been in love with her next door neighbor, Masao, ever since he moved in with his family five years earlier, but he always treats her like he would a younger sister. Masao's younger brother, Yukio, on the other hand has secretly harbored feelings for Chie for almost as long of a time. One day, Masao suddenly announces his engagement to a girl she'd never met. Will this really be the end of her unrequited love?

This was also adapted into an audio recording in the late 80's.