They did pretty well...didn't they? So did we in America...until we "diversified" our gene pool...before we contaminated it with African DNA. Now we're #25 in the world.

ALL the "royal families" married from within a small group of related rulers.

From your own link:

" It seems as if the Ptolemys would have then produced a number of offspring with genetic disorders, but none appear to have significantly suffered from inbreeding. Nevertheless, inbreeding worked in their favor and helped keep the rule of Egypt in the hands of the Ptolemys for almost 300 years. "

The whole "inbreeding" thing is propaganda to KEEP families from staying genetically pure. If you have no genetic diseases you won't CREATE them by having a child with your sister. It's when the family DOES have genetic diseases that they keep coming out when relatives who already carry it, have kids.

Inbreeding doesn't create monsters...unless the monster genes are already present.

Incest is against the law for social reasons not biological.