Ameiro Banyuu Inryoku

From DMP:

Since their parents moved overseas, Toru, Hajime and their little brother Futaba have lived together alone in the Katsuragi house. But then suddenly their problematic cousin Takara arrives!! As he enters the three brothers' lives are in an uproar, for some reason Takara doesn't try to get close to them, and his childhood friend Toru feels unbearably snubbed. When Toru tells him so, Takara quickly shows him a gentle smile, but what kind of dangerous schemes hide behind that expression--!? Palpitation and anticipation under one roof.