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.
Roman emperor then create a rival lineage of Roman emperors in western Europe, the Holy Roman Emperors, which ruled the Holy Roman Empire for most of the period between 800 and 1806. These Emperors were never recognized as Roman emperors by the court in Constantinople. Modern historians conventionally regard Augustus as the first Emperor whereas Julius Caesar is considered the last dictator of the Roman Republic, a view having its origins in the Roman writers Plutarch, Tacitus and Cassius Dio. However, the majority of Roman writers, including Josephus, Pliny the Younger, Suetonius and Appian, as well as most of the ordinary people