Julia Drusilla was one of the daughters of Germanicus and Agrippina Senior. She was also the sister of Nero Caesar, Drusus Caesar, Caligula, Agrippina Junior and Julia Livilla. Born in 16 AD, she was only three when Germanicus died in Antioch. Drusilla and her siblings were brought back to Rome by their mother, from where they all lived in Germany. The children were raised by Agrippina Senior, with the help of their paternal grandmother, Antonia Junior.
During 26 AD, the Praetorian Prefect, Sejanus, essentially was running Rome and the administration of it, thanks to Tiberius delegating so much responsibility to him. Although Sejanus was not in line for succession to the throne, he used his power to eliminate possible challengers. Agrippina Senior, Nero Caesar and Drusus Caesar were all arrested, exiled and died under mysterious circumstances. In 31, Caligula accepted the invitation to live with Tiberius on the island of Capri, where the emperor had been since turning over duties to Sejanus. Tiberius had one grandson, Tiberius Gemellus, who joined the emperor and Caligula in Capri that same year. Tiberius named both as joint heirs, although Caligula was the favored of the two.
In 33, Drusilla married Lucius Cassius Longinus, a friend of the emperor Tiberius. Longinus had been consul in 30 and in 37, Tiberius appointed him to a position of commissioner. When Tiberius died in March of 37, Caligula was named emperor and adopted Gemellus. He also ordered Drusilla to divorce Longinus and marry his friend Marcus Aemilius Lepidus, who was said to be romantically involved with not only Drusilla, but also Julia Livilla, Agrippina Junior and Caligula himself. Later that same year, Caligula became terminally ill and named Drusilla his heir, making her the first woman to be the heir to the Roman Empire, likely to keep the Julio dynasty intact through any children she would have. However, Caligula recovered, but the near-death experience made him paranoid. In 38, he had Gemellus executed for allegedly being part of a plot to assassinate him because his breath smelled of antidote against poison, which was considered treason. The apparent truth, however, was he was taking cough medicine, but it was a good enough excuse to rid Caligula of a potential rival just in case.
It is well-documented Drusilla was the favorite sister of Caligula, who was four years older than her. There is much written about their incestuous relationship, which continued into adulthood, but there are some different hypotheses about why. Often, historians use the relationship to demean Caligula, but some propose the emperor was thinking along the lines of the Ptolemies and wanted to jointly rule with his sister. This went against Roman traditions, however and Caligula was already married to Lollia Paulina and Drusilla was married to Lepidus. Even at dinner parties, Drusilla was seated where Paulina should have been, inferring Drusilla was actually his wife and the female head of his household.
On June 10, 38 AD, Drusilla died, likely of a widespread illness in Rome at the time. Caligula was devastated and had her deified, despite not even Livia, the wife of Augustus, having yet been given such honor. He also forced the public to mourn for an extended period of time. Drusilla’s widower, Lepidus, was the most likely heir to Caligula, and was said to frequently be Caligula’s bed companion, but the following year was executed, again for allegedly conspiring to murder the emperor.