For example it's possible to marry your close kin which gives you an opinion bonus with other characters. This includes father-daughter, mother-son marriages. I know that royalty routinely practiced marriages between cousins, but Zoroastrians are the only ones I've read about actually practicing marriages between immediate kin. Did it ever actually work like this in real life? Also are there any other religions, societies, where incest wasn't considered a taboo? On the other hand, what is the MOST incest-averse society that historians know of?
I asked myself the same question while playing...but I knew about the wonderful encyclopaedia iranica, a very complete and up to date work on the iranian world. Here is the article on the subject
The most important element: it seems to be an incestuous tradition at least on some part of the zoroastrian society, but it was not only for brother and sister but also father and sister or mother and son. The confirmation elements are coming from christians and jews living on the limits of the persian world who are obviously at odd with such practice, but his extend among society is unknown. In addition we only have certain and clear sources only for the 16th century onwards but at the time (and later) it was defined as a marriage between first cousins, a practice that is seen quite often even today in multiple societies on the planet so it would certainly not surprise foreigners.