"One of the other sources for Spike was an Emperor of Rome called Caligula, who was a sick and twisted man. He had an affair with his half sister - her name was Drusilla. So who knows the history of Dru and Spike!" JM

Q: How do you come up with the names of the characters, and does Pike from the movie have anything to do with Spike?
Joss Whedon: No. I came up with Spike and Dru and I was all into that, and then somebody reminded me several months later that I had a character named Pike, and I was like, "OK, I suck" - that was another one of those exciting moments. I love the name "Spike", I always love it in pair with a woman; "Drusilla" I took from Emperor Caligula, his wife-slash-sister, whatever she was, 'cause I loved I, Claudius when I was a kid, and so I was very into the idea of this very twisted relationship, and "Drusilla" seemed a perfect classical match to "Spike". The first characters. it takes us a very long time to name characters: it's about finding a name that, most importantly, doesn't sound like any of the other characters, so they really have their own feel every time you hear their name. (Source)

"He lived in habitual incest with all his sisters and at a large banquet he placed each of them turn below him, while his wife reclined above. Of these he is believed to have violated Drusilla when he was still a minor, and even to have been caught lying with her by his grandmother Antonia, at whose house they were brought up in company. Afterwards, when she was the wife of Lucius Cassius Longinus, an ex-consul, he took her from him and openly treated her as his lawful wife; and when ill, he made her heir to his property and the throne." (Source)

"His parents had obviously failed raising him because according to some scholars Gaius slept with his sister Drusilla at the age of thirteen." (Source)

"In fact, he loved his sister, Drusilla, so much that he had an affair with her before he had even come of age. (...) He was married several times but always came back to his favorite of his three sisters, Drusilla, whom he forced to live with him as his wife. The stories say that, when she became pregnant, he was so excited that he could not wait for the baby to be born and disemboweled Drusilla to get to the unborn child. Whether that is what happened or not, her death brought with it the loss of any control over Caligula that existed." (Source)

38 CE: (...) Later in the year Caligula's favorite sister, Drusilla, died; he had been so close to her that there were rumors of incest. In the manner of the eastern monarchs, Caligula had Drusilla deified; she was the first Roman woman ever officially declared a deity, but her divinity did not survive his reign because he had so egregiously flouted Roman precedent (in contrast, when Claudius had Livia deified, he emphasized her role as “diva Augusta,” wife and mother of emperors). Caligula also minted coins in Drusilla's honor. (Source)

Some accounts say that Caligula was very cruel and vicious with not only Drusilla and his sisters, but also with others. About six months after he became Emperor of Rome he fell very ill and afterwards started to behave strangely and at times 'crazed'. Some think that he might have had a mental illness. (Gathered from my readings. GP)

CALIGULA XXIV-XXV, XXXVI (it might be down or removed)

Caligula and Drusilla

Caligula: Historical Background (with info on Drusilla)

Coins with images of Caligula and Drusilla