Incest 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. 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). Emperor Claudius, after executing his previous wife, married his brother's daughter Agrippina the Younger, and changed the law to allow an otherwise illegal union.
Agrippina the Younger cousin, Drusilla's widower Marcus Aemilius Lepidus, were involved in a failed plot to murder Caligula, a plot known as the "Plot of the Three Daggers", which was to make Lepidus the new emperor. Lepidus, Agrippina and Livilla were accused of being lovers. Not much is known concerning this plot and the reasons behind it. At the trial of Lepidus, Caligula felt no compunction about denouncing them as adulteresses, producing handwritten letters discussing how they were going to kill him. Lepidus was executed. Agrippina and Livilla were exiled by their brother to the Pontine Islands. Caligula sold their furniture, jewellery, slaves