Engage (IKUEMI Ryou)

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?

Note: This series also inspired an image album recording in the late 80's, with music composed by Haruo Kubota and lyrics by Maki Kimura (alongside tracks inspired in Machi to Hito no Romansu and Ishindenshin no Otsuki-san).