Kin no Ouji to Gin no Ou (2006)

Blurb:
Getting adopted by a rich family might be every boy’s dream, but for Yuuki, this unlikely reality comes with a price of its own. After the death of his mother, Yuuki discovers he’s the illegitimate son of a wealthy businessman, also deceased. The remaining Watase family consists of two older half-brothers: Masumi, a kind-hearted gentleman near Yuuki’s age, and Gai, a stoic workaholic several years his senior. Trying to fit in with a different social class proves difficult enough for Yuuki, but when hidden feelings beyond simple brotherly love start to emerge, confusion and angst truly begin to set in. Can such feelings, even between half-brothers, ever be accepted? If so, is it the outgoing Masumi who holds the key to Yuuki’s heart, or the ill-tempered Gai who will teach him the true meaning of love?

Source: www.anisearch.com/manga/4524