Germanicus had two daughters named Julia, this being the second (it was not unusual for Roman sisters to share a name and to be referred to, for example, as Julia Major and Julia Minor). Claudius considers the name (one of the most common female names in Rome at the time) to be "unlucky" because of the association with Julia Augusti Filia, but states that he always called his niece "Lesbia" (she was born on the island of Lesbos), a name made equally scandalous by Catullus's poetry.
His sudden death in Antioch was never fully explained - there was a strong suspicion that he had been murdered on Tiberius's orders, and that the Governor of Syria, Calpurnius Piso, with whom he had quarelled, may have been implicated. Once again, Graves's fiction (in this case, the story of the witch, Martina, employed by Piso and his wife to terrify the superstitious Germanicus), though loosely based on Tacitus's account in The Annals, shines a light on a corner of the past that history has left dark.
The return of Germanicus's widow, Agrippina Major, to Rome with his ashes (via the port of Brindisium) is described by both Suetonius and Tacitus. Germanicus's death and funeral represented such a traumatic moment in the history of Rome that they became a significant theme in 17th and 18th Century European art.
Gnaeus Calpurnius Piso (43 BC - 20 AD) went on trial for the murder of Germanicus. He died before the trial could be concluded, supposedly by suicide, although there were suspicions that he had been murdered. His wife Plancina was acquitted, probably following an intervention by Livia.
The main source here is Suetonius, who tells us in The Twelve Caesars how Tiberius, at his retreat in Capri, paid male and female prostitutes to "engage in filthy threesomes in his presence," and how he had "boys and girls dressed up as Pan and the nymphs" soliciting outside caves and grottoes in the grounds of his villa.
There was also a story that he "trained some boys of tender age, whom he called his little fishes, to slip between his thighs when he was swimming and provoke him playfully with their licking and biting." It is unclear how much of this was made up, but Graves is quite restrained in his use of the source material.