Definition - What does Incest mean?
Incest refers to any sexual relations between immediate family as well as family close enough that marriage is forbidden by law. Since marriage laws differ between provinces and states, what might be considered incest may vary between regions.
Fantasies around incest are common, although far fewer people act them out. Like most fantasies, this one involves breaking taboo.
Kinkly explains Incest
For relations to be incestuous, the individuals must be close enough related that marriage is not permitted by either law or custom. In some areas, this includes all immediate family as well as cousins. The marriage of first cousins is permitted in some areas, though, generally, the marriage between first and second cousins is considered incestuous.