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 Elder ties to the emperor. Historian Richard Alston says it is likely that either Tiberius or Livia were behind the exile of Agrippina's half-sister and the death of Postumus. He notes the death of Agrippina's mother, who starved herself to death amidst her exile in AD 14, linking her death to Tiberius' disdain for her. Agrippina was vocal about her feelings claiming that Germanicus was murdered to promote Drusus the Younger as Tiberius' heir, and worried that the birth of the Younger Drusus' twin sons would displace her own sons in the line of succession. Her fears proved to be unfounded,