Brother-Sister Incest/Tabletop Games


 * In Deadlands, the Whateley family tree seems to drop nothing but bad apples. Part of the reason for this is the "selective breeding" instituted by the clan's otherworldly patron. Most of the residents of Gomorra, California can't figure out exactly what the relationship is between Nicodemus and Delores Whateley. Some think they're siblings. Some think they're married. Both are right. Squick. (Bonus prize: reading the Whateley Family Bible, complete with a family tree in the front, has driven some folks insane.)
 * In the Ravenloft supplement Dark Tales & Disturbing Legends, a highborn woman magically transforms herself into the guise of her brother's bride so he won't "pollute" their bloodline with a commoner wife.
 * Dungeons & Dragons
 * In the 3.5 manual Exemplars Of Evil, among the featured villains are the Tolstoff siblings - Edgar, an insane cancer mage, and Katharin, a seductive enchantress - who serve an Eldritch Abomination known as Kyuss and (more concretely) their Sealed Evil in a Can grandfather who promised them to protect their forbidden love if they could help him out of his confinement. Their desperate love for each other would make them pitiable and somewhat sympathetic - if the extremes they are willing to sink to didn't send them straight into Complete Monster territory.
 * The principal draconic deities are Tiamat the Dragon Queen and her brother Bahamut. Tiamat is the goddess of evil dragons and Bahamut is the god of good dragons. They are fierce enemies, but Tiamat's hatred for her brother has developed into a depraved lust over the thousands of years they've been fighting. The feeling isn't mutual, however.
 * The 4th Edition lore might actually make this a Selfcest situation: both deities are two sides of Io, a more powerful deity who was slain and sundered during the Dawn War, the two halves of his body becoming individual entities - Tiamat embodies Io's greed, selfishness, and vanity, while Bahamut embodie his wisdom and honor (but sadly, also Io's arrogance and hubris, which is likely what got him killed in the first place). Whatever the case, their rivalry is... complicated to say the least.
 * In the Eberron campain settingm the god known as the Devourer raped his sister Arawai, resulting in the birth of the goddess known as the Fury.