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 first marriage to praetor Tiberius Nero, and was the Emperor Tiberius's younger brother and Augustus's stepson. In the year 9, Augustus ordered and forced Tiberius to adopt Germanicus, who happened to be Tiberius's nephew, as his son and heir. Germanicus was a favorite of his great-uncle Augustus, who hoped that Germanicus would succeed his uncle Tiberius, who was Augustus's own adopted son and heir. This in turn meant that Tiberius was also Agrippina's adoptive grandfather in addition to her paternal great-uncle. Agrippina was born on 6 November in AD 15, or possibly 14, at Oppidum Ubiorum, a Roman outpost on