Apologies, the link didn't work, but I extracted this, which may answer you question.
It is likely the marriage between siblings in ancient Egypt in the
pre-Ptolemaic periods has more to do with the ritual relationship
of Ra with Hathor, who served as mother, consort and daughter all
at the same time. Thus, the daughter of the king /sAt nsw/ provided
the continuity symbol of the kingship's divine nature,
I read somewhere that this was more widespread in ancient Egypt than just among the pharaohs. As far as the rulers go, it was probably just a way of making sure power didn't pass outside the family. The upper classes these days are still somewhat inbred.
incestual marriage and child bearing was much more widespread than just the ancient civilisations, and "saving" the purity of the blood, or even keeping land/property "within the family" was common up until very recently (19th century etc)
Why Not? There were far fewer mates to choose from, the further back in History one delves. The Royal Heads of Europe and the rest of Earth, too, were, and are (depending upon the Degree) incestuous. It has been a means, however detrimental, of keeping the Blood Line Pure.
Incest actually happened a lot of places, not just Egypt, (or the back hill of Southern United States). It was thought, just like with dog breeding today. That to strengthen the blood line, breed two of the best together. There are dog breeders now that breed related dogs for the same reason. Recent testing has found that it doesn't cause birth defects on humans. Still a little gross though. Hope this rambling helped.