incest
n. sexual activity between family members, whether related by blood or not. Some form of incest taboo, or prohibition in custom or law against incest between first-degree relatives (e.g., father–daughter, mother–son, brother–sister), is found in nearly every society. In some societies, sexual intercourse between cousins, uncles and nieces, or aunts and nephews is also prohibited; in others, it is permitted. —incestuous
adj.