Meigyoku no Almaine

With a king falling in love with his mother at first sight, Almaine became a member of the Fimmelhausen dynasty. However, one day the ten-year-old learned of his mother's death, had become keenly aware of the royal family's repulsiveness from an incident with Hiltrud, his beautiful elder sister, and flew from the palace. From there, he spent eight years roaming the continent when his matured younger sister, Naria, appeared before him as a knight. Reluctantly, Almaine would return to the palace for a time, but would be met with the harsh fate of the royal family, captive to their blood...

(Source: Baka-Tsuki)