Short answers, pulled from the story.
Cleopatra VII wed two of her younger brothers, Ptolemy XIII and Ptolemy XIV, while Tutankhamun married his half-sister Ankhesenamun. This state-sanctioned policy was practiced by the Ptolemaic dynasty and earlier pharaohs to prevent the royal blood from being diluted by outsiders.
Tutankhamun's mummy shows evidence of a clubbed foot and a cleft palate, which are physical manifestations of the inbreeding depression that plagued the family for generations. These severe genetic defects contributed to the eventual collapse of the dynasty.
King Gwangjong wed his half-sister Daemok in the 10th century as part of a calculated political strategy to prevent the dilution of royal authority. This union mirrored the strategies of ancient Egypt and the Inca Empire to keep wealth and power within the immediate family.
Sixteen adults were charged with hundreds of allegations of abuse in the Goler clan case, which involved fathers, mothers, uncles, and aunts engaging in sexual acts with children as young as five. Research suggests that sibling incest is the most common form of intrafamilial abuse, yet it is rarely reported.
Offspring of sibling or parent-child unions face 20 to 36 percent rates of death or major disabilities due to the increased probability that both parents carry the same deleterious recessive allele. Studies have shown that children of first-cousin unions have a birth defect rate twice that of unrelated couples.
Sweden allows marriage between half-siblings under certain conditions, and the Netherlands permits marrying a nephew or niece with government permission. The United Kingdom and the United States have different laws regarding first-cousin marriage, with some states prohibiting it and others allowing it.