Based on the erotic game by Black Lilith.

Kyle, the protagonist of this story, is one of the 7 heroes who defeated the evil overlord. After that battle he starts a conquest of the continent in order to unite the humans who live on it.
Standing against Kyle is only his younger brother Syll, who is also one of the seven heroes. Kyle stole his throne, but failed to kill him.
One day, another of the 7 heroes appears before Kyle. She is Nina, an elven princess. Kyle knows that Nina and Syll are lovers, and a dark desire begins to burn inside of him...

(Source: DLsite English)