In AD 295 incest was explicitly forbidden by an imperial edict, which divided the concept of incestus into two categories of unequal gravity: the incestus iuris gentium, which was applied to both Romans and non-Romans in the Empire, and the incestus iuris civilis, which concerned only Roman citizens.Therefore, for example, an Egyptian could marry an aunt, but a Roman could not.
Many European monarchs were related due to political marriages, sometimes resulting in distant cousins (and even first cousins) being married.
Despite the act of incest being unacceptable within the Roman Empire, Roman Emperor Caligula is rumored to have had sexual relationships with all three of his sisters (Julia Livilla, Drusilla, and Agrippina the Younger).
The taboo against incest in Ancient Rome is demonstrated by the fact that politicians would use charges of incest (often false charges) as insults and means of political disenfranchisement.
In Norse mythology, there are themes of brother-sister marriage, a prominent example being between Njörðr and his unnamed sister (perhaps Nerthus), parents of Freyja and Freyr.
Loki in turn also accuses Freyja and Freyr of having a sexual relationship.