Incest

"Brothers and sisters? Where I come from that stain is very difficult to wash out."

- Olenna Tyrell to Tywin Lannister

Incest, the act of engaging in sexual intercourse with one's direct family member (siblings, parents, offspring) is a great cultural taboo both in the Seven Kingdoms and the lands beyond the Wall.

Alongside kinslaying and the violation of guest right, incest is proscribed by every major religion in Westeros. Children born of incest are considered abominations

Known instances of incest

 * The members of House Targaryen, like their ancestors of the Valyrian Freehold, often married brother to sister to keep their bloodline pure, though it also caused insanity on some of them. Despite the Faith of the Seven proscribes incest, it turned a blind eye to the practice of the royal family.
 * Queen Cersei and her brother Ser Jaime Lannister have carried an illicit romance since childhood. Cersei's children: Joffrey, Myrcella, and Tommen, are all born of their affair.
 * The wildling Craster takes his daughters as wives and does the same on the daughters he sires on them.