We know the Ptolemaic dynasty practiced inbreeding to avoid problems of succession and keep power within the family. We have seen cases like this in other royal dinasties such as the Habsburgs in Spain, culminating in the infamous Charles II. However, Charles was the product of repeated marriages between cousins and uncles and nieces; Cleopatra's ancestors married their siblings numerous times, and there's not any case of malformations that's as famous as Charles, despite the odds that they could have been even worse. Cleopatra's famous beauty, wit and intelect seem to have escaped those odds, or so history says. Or is there any account of the probable genetic defects the Ptolemaic dynasty would have suffered?
I have written extensively on this very topic, and I recommend you check out these older answers of mine:
In societies where incestual marriage with very close degrees of relation was tolerated/celebrated (eg; brother-sister pairings in Egypt) did people ever notice or point out that the children of such pairings seemed to die more often than ones produced from non-incestual marriages? and a follow-up discussion in a related thread.
Additionally, you may find this post to be of interest:
Hopefully this helps and if you have any follow up questions you can feel free to ask!