Azrael-ka no Jinan wa Han ma

The second son of the Azrael family was unable to use magic and supported the family as a househusband. One day, his instincts as a demon awakened and he found out that he was actually half-demon and half-human. The eldest son keeps the secret and supports him, but in order to suppress his demonization, he needs a supply of magical power through sex...this leads to the eldest son's obsession and the twin brother's desire for possessiveness. The relationship between the three of them, who should have been brothers, slowly, sweetly breaks down.