You're right, maybe if the devs can make sure that those hellenic/eastern syncretic cultures can marry between siblings even outside of their cultural area, it will be more correct..Me, I agree with the fact that inbreeded marriage should occur only in the area influenced by Egypt and Iranian civilisation( so from Asia Minor up to Bactria)
Even though half Brother Sister wedding occured between Ptolemy Keraunos and her half sister in Macedonia , it was more of a move to seize power for Ptolemy and the wedding between Alexander II and Olympias II of Epirus was more of an exception than a rule (Pirrhus had been welcomed to Alexandria and befriended Ptolemy and later bring the custom with him in Europe)
The house of the Antigonids, the dominant power over the Greek peninsula never used incestuous marriage.
So for example I'm the Antigonid king of Atropatene and my religion is a mix of iranian and hellenic, I want to marry my sister who is pure hellenic and she reside in Macedonia, I can do that, if the king of Macedonia agree on that, clearly. Imho is simple to implement.