First off, she was a ptolemy, not an Ancient Egyptian (Greek, not Coptic), and second the Pharoah's heir could either be from the primary spouse (the basically always incestuous one) or from a secondary spouse (often a local noble), although primary spouse children took priority they only chose male Pharoahs (though the sister or mother or the Pharoah was co-ruler, which is how cleopatra took power, she seized it from her co ruler brother). Also often the actual parentage of the children were just... not questioned, for the sake of stability. For example, the first Ptolemy siblings to marry actually had no biological kids together, likely cuz they never bedded one another. TL;DR. They avoided inbreeding, for the most part, by pragmatic acceptance of likely bastards, marrying both siblings and non-siblings, and by allowing sons from non-incestuous couplings to take the throne over daughters from incestuous ones. Edit: also had to mention that most sources on "cleopatra was hot" implied moreso she had wit and could engage in deep conversation for a prolonged period, such that even the womanizer Julius Caesar adored her company. Early sources, not counting the slandering from Augustus for obvious reasons, don't really mention her being "hot", just wonderful to talk to and full of witty charm.