Ookami Wa Koi Ni Naku

From Flipped Switch Scans:
Deep in the mountain range, lives a clan that coexists with wolves. Children with black hair are called “Wolf” and children with white hair, although rarely born, are called “White Hart”. A “White Hart” is a symbol of prosperity, and these children are obligated to marry into the Royal Family.
Sara was born a “White Hart”, and from young, had feelings towards his companion, Alexis, the King’s son. But he knows that Alexis prefers his twin brother, Nara. Although determined to be a substitute for the person he loved, Sara could not continue on suppressing his feelings.
To love one person for all eternity, this is the fate and the story of the “White Hart” and “Wolf”.