Yasoukyoku夜想曲 ; 浪漫琴心夜想曲 ; Nocturne (TAKAMURE Tamotsu)

Shou is the illegitimate son who is mistreated by his family, while Takashi is clearly the favored son. Shou is being "used" by Takashi, but Takashi's fiance has fallen for Shou. Feels like a shoujo manga, but with a little bit of yaoi mixed in. There is a sequel to this manga called :Zensoukyoku it has not been scanlated yet.