Himekishi Lilia

Himekishi Lilia AKA Princess Knight Lilia is a popular Japanese hentai video-game and ongoing anime series produced by PIXY. Set in the realm of Leuven, the king is overthrown by his own son - Dirk with the help of the demon world. In return, he must pledge the magic power from his sister Lilia's orgasms. With the help of the demon dominatrix, Kiriko, Dirk begins corrupting his young sister along with his mother, the queen Leila, with all manner of depravity.

Provides examples of

 * Brother Sister Incest - Dirk has his wicked way with Lilia.
 * Deal With The Devil - Dirk makes a pact with the demons to become king, in return for Lilia's magic orgasms.
 * Heroic Willpower - Depending on the player, Lilia can overcome her horrific torture... or become a victim of Rape Is Love.
 * Hot Shonen Mom - Queen Leila.
 * Naughty Tentacles - It would make Urotsukidoji blush!
 * Nightmare Fuel
 * Parental Incest - Dirk rapes his mother, Leila. Lilia is raped by her uncle in episode 4, and Lilia also enjoys a romp with Leila in episode 5.
 * The Caligula - From coveting his own sister, to holding depraved orgies, Dirk fits the bill.
 * Squick
 * With Great Power Comes Great Insanity - Dirk becomes king and unleashes all hell - literally!