The Lannisters are the richest family in Westeros, making their primary home in Casterly Rock. They are led by Tywin Lannister, who was once the primary adviser to King Aerys Targaryen, and later helped conquer King's Landing for Robert Baratheon's rebellion. His three children are Cersei and her twin brother, Jamie, and the dwarf Tyrion, who is by far one of the richest, most interesting characters in a story chockfull of them.

As mentioned above, Cersei's son Joffrey, as well as the other Baratheon children, weren't actually fathered by King Robert. They are instead the product of an incestuous relationship between Cersei and her brother Jamie. While their relationship is certainly taboo and would result in the loss of the Iron Throne if it became common knowledge, incest amongst royal families has its precedent in Westeros, as it was practiced amongst the Targaryens for generations.