Tsuki no Hikari mo Todokudarou

From Fantasyshrine:

Five years ago in summer, Takuro's older brother, Kotaro, a high-school teacher, brought his student, Hiro, to Takuro's apartment. The three of them started living together happily.
Until one night, Takuro discovered that Kotaro and Hiro were actually lovers. Although he felt hurt, angry, and betrayed; Takuro realized he had also fallen in love with Hiro.
However, tragedy struck when Hiro's older brother, Reiichiro, finally located Hiro…