The disappointingly partisan, fake-news Protestant movement
The suggestion that Sforza was unable to perform was predictably humiliating for him and he responded with a counter-claim on incestuous relations within the Borgia family. There is no reason to think this was anything more than mud-slinging, and indeed as mud-slinging it had some metaphorical weight. The infamous emperors Nero and Caligula had been accused on incest in Suetonius' Lives of the Caesers, the latter with his three sisters; Suetonius, a classical writer, was a fashionable source for Renaissance authors and editions of his work were widely available in Rome. The household, moreover, was a popular metaphor for government: a man who tolerated or indeed fomented sexual perversity at home was bound to be a poor ruler when it came to the state. None of that, however, mattered to later Protestant writers who delighted in taking literally these tales of popish outrage.