The protagonist's father makes a deal with a devil. Shortly thereafter, a succubus visits the hero's home. She was there to fulfill his father's contract. However, she falls in love with our hero at first sight, and decides to live with him and his stepsister instead. Soon the succubus' mother comes knocking, and even she wants to move in on the action! Of course, being succubae, they need to get laid once in a while to keep going--might as well give them what they want!

[From White Masq]