Pharaohs did publicly and with swag. It’s well-documented.
The other well-documented thing is Greek historians’ (specially Herodotus) hatred and false historiography of Persia.
That tradition still continues with movies like 300, that blatantly distort history.
Some things haven’t really changed
Ptolemy was living in Hellenistic Egypt, and in this Egypt the practice of marrying siblings had spread amongst the noble non-royal classes. This is well documented.