Originally Posted by
oudysseos
You have to stop and consider for a moment how likely it is for widely divergent societies, some of them quite xenophobic, to acquire Family Members from unrelated ethnic groups. Yes, very gradually, Roman generals, magistrates, senators and even emperors came from originally non-Roman peoples, but this process took a very long time- in most cases, longer than the game lasts. And the Romans were very liberal about assimilating new peoples compared to many others such as the Carthaginians. Our guide here must be common sense and plausibility, together with historical examples. If you can think of any serious instances where, say, a Gaul became part of the Numidian family tree, then tell us about it.
Don't confuse Exogamy with ethnicity: Antiochus I Soter may have had a Persian mother, but he was not considered a Persian by any means, and his children and descendants were Macedonians.
Of course there are grey areas: the Kings of Pontus were well intermarried with the Macedonian lines- but that didn't really make them Maks, eh?
In the end it depends on each faction and how likely we think they would have accepted truly foreign men into their power structure. But bear in mind that a son inherits his fathers ethnic trait, so that adoptees, men of the hour, and husbands are the only way to introduce new ethnicities into a factions family tree.