Isuke's unnamed brother died while Isuke was young, at the neglect of their parents. Isuke appears to show some kindred emotions for him, claiming he was all she had before she was rescued by Eisuke. However, she also states that "the weak die, that's all." The only visuals of her brother mostly show him postmortem. In the anime, he is almost completely shadowed over, though it is possible he has the same hair color as Isuke.
The Inukai family grave
In the doujin, it is revealed that he and Isuke's original parents are buried under the Inukai family grave despite not having the surname, which is Eisuke's. Isuke appears to visit frequently, and Haruki brings flowers after she and Isuke are engaged.