My comments in this thread should help you out a bit. The short version is that there's absolutely zero evidence to suggest that incest was practiced outside the royal family, and even then it was nowhere near as prevalent as some people would like to believe. It only because common in the last few generations of the Ptolemies--see here. Nor was most of this royal incest, with the exception of a handful of kings, generally between siblings, but instead tended to be brothers marrying each others' wives or daughters. There have been some attempts in the past to provide evidence that Ptolemy XII had some sort of mental illness, but there's really not enough in our texts to back that up--from our texts it would just appear he was a flute-playing greedy little shit