Gokudou Darling

From Nakama:

When his mother dies in a sudden accident on a honeymoon after her remarriage, the destitute host, Tatsumi, has no idea that her new husband had been the oyabun of a yakuza gang. But while Tatsumi continues to refuse to accept his inheritance, the new leader of the gang, and Tatsumi's step-brother, Yoshimune, appears. He welcomes Tatsumi into the family by force. Gradually, while living with Yoshimune who won't leave him alone for some reason, Tatsumi starts to...?! A manly story of the defensive and offensive love strategies of two brothers not related by blood.