The rape you referred to is a later Roman invention, the Roman's were not averse to a bit on incest, in fact they positively encouraged it for certain classes.
Probably why so many of them were lunatics.
Yup, Ovid fabricated it in his "Metamorphoses." But the classical Greeks also shifted away from Medusa being ugly/hideous. The earliest concrete evidence is Pindar's Pythian Ode 12 from 490 BCE ("fair-cheeked Medusa"), and she was depicted as attractive/beautiful on many vases starting about 40-50 years later.
Roman society was simultaneously fascinated and disgusted by the idea of incest. It was considered "incestum" (not pure/impure) behavior. Generally you couldn't get away with it unless you were powerful...if you weren't, you got socially ostracized, or worse. And among the elites alleging your political opponent practiced incest was very damaging if there was anything that appeared remotely inappropriate.
Even at that it applied mostly to your direct ancestors, descendents, and siblings including half- and step-siblings or by adoption. But once you got to third-degree consanguinity (ex: uncle+niece) it started to get distasteful-but-allowed, and more acceptable as the degree of separation got larger.
"Incestum" covered a whole lot of other things as well. I remember something about a Roman senator getting charged with incestum by sneaking into an female-only religious ceremony dressed in drag. He didn't do anything sexual, just snuck in to witness it.
