This is not really in the vein of incest, but it doesn't warrant its own thread, and fits the disturbing aspect of this thread to a 'T', IMO.
My buddy the DM, in order to show us the depths of depravity of the villain, favored us with this encounter:
Fair warning: This is Very Disturbing (to me and the rest of the gaming group anyway).
[spoiler]"You enter the main level of the house and find it silent except for the crackle of a fire. As you investigate the house you find dead members of a family. The sound of the fire seems to be coming from the kitchen. Once there you see what looks like a zombie of a little boy, maybe 5 or 6 years old, turning the body of his little sister on a spit over the flames."[/spoiler]