The family adopted almost all the customs and rituals of the Pharaohs to give the population the sense of continuity and avoid insurrection. That included incestuous marriage in order to keep the bloodline "pure". Cleo had her young brother killed and avoided that, and her only issue was the (supposed?) son of Julius Caesar, Caesarian. Octavian made sure he didn't make it either.
Yup....thus I’m now been better schooled! Thanks! :)