I've written on the idea of incest in the royal family before (https://www.reddit.com/r/AskHistorians/comments/wgw098/what_did_the_regular_people_of_ancient_egypt/ij48wz5/?context=3), but there is (as always) a little more to be said.

Royal incest was not just acceptable, it was expected. The king was human incarnation of and successor to the god Osiris, and Osiris had married his sister Isis. So the king too would marry his own sister (and there are plenty of other ideological and philosophical reasons why the king ideally would marry his sister in my comment above). Nonetheless, this did not always happen - incest is often a taboo in human cultures, and the same was true in Egypt (as we can see from the fact that normal human Egyptians didn't engage in it). So while all Egyptian kings did marry their sisters - to allow a sister to marry outside the family would have been to allow power to be diluted - they almost certainly did not have sex with all of their sisters; by marrying them they enabled them to become part of the kingship and thus fulfill the king's feminine duties for him. In addition, the kings were also polygamous, and so many of their wives would be drawn from the nobility or foreign rulers - though again, in the international diplomatic marriages of the second half of the second millennium BC the Egyptians did not let their princesses marry foreigners.

In short, incest was reasonably common in royalty throughout the pre-Ptolemaic period, but was not universal and was more common in some periods - generally those associated with strong centralised control of the country - than others. So at the end of the 6th Dynasty, when Egypt was about to collapse into the First Intermediate Period, we find the kings married to noble rather than royal women, indicating the relative power of regional governors compared to the king. It can be difficult for historians to assess the statistics for certain: as I mentioned, the king's wife was ideally Isis (the king's sister), which means that she is often described as such even if she wasn't related. Biologically, genomic sequencing does allow us to better understand the relationships between royal families, though it's dependent on us finding and testing the right mummies, which is not necessarily easy.

In fact, the Ptolemaic period is when incest was probably most practised, and certainly most messily practised - the family trees of the later Ptolemies are a nightmare, both in terms of complexity and general ickiness. And certainly this is true of normal Egyptians: before Ptolemy II married his full sister Arsinoe II people didn't marry their siblings; after this it becomes a very widespread practice. So, if we limit the memes to just royalty, they are (relatively) accurate throughout Egyptian history; when applied to Egypt's entire population, they're only accurate from Ptolemy II onwards.