Keeping it in the Family

Alfonso VI of León (1066, Iberia in Pieces) starts as lovers with his own full sister, Urraca. Keep producing children with her until one arrives with the Inbred trait - full-sibling children roll the dice hard in that direction. NB the wiki flags this achievement may not be available for ruler-designer characters, so use a historical start like this one. The Things We Do for Love is sitting in the same save.