Hitotsu Yane no Shita no Kedamono

From Canis Major Scanlations:

Daigo's going to be a university student starting from spring. He has two male cousins who are identical twins, called Shin and Jin. Daigo has always been secretly in love with the kind Jin who's always treated him like a younger brother. However, his feelings were exposed by Shin, who's always doted on him as well. Under pressure by Shin who said to him “Choose me,” although he resisted at first, looking at the same face as the person he likes and hearing the same voice, his heart can't help but beat faster…?!