I made an earlier post involving this so I'm copying out the relevant paragraph (in case anyone read the earlier post): Yes and no. Historically, incest in Zoroastrian culture was technically seen as pious. The term for incest was xwēdōdah, considered to be amongst the most pious actions a person could do. It was specifically between siblings, father/daughters and mother/sons. It's unclear how commonly it was actually practiced by normal, ordinary people, however. We have limited legal records involving cases where xwēdōdah is mentioned, but there are only a few examples- and usually the xwēdōdah plays a central part (for example, if a man has bequeathed one piece of jewellery to his next wife and another piece of jewellery to his daughter and if his daughter becomes his wife, is she entitled to both pieces? Stuff like that was what the legal records mainly involved). If the problems arising with xwēdōdah necessitate going to court, I would argue it was unlikely to be common. If some very basic things related to marrying your daughter need legal help, then surely they would not be so basic as we assume. To a society where xwēdōdah was not just legal but common, these issues would arise more commonly and have a commonly known solution- there wouldn't be people arguing over it in court, the same way we don't have people arguing that they should inherit the thing given in a will if there is no reason to contest it at all. If it is given in the will and all seems well, then the person inherits the object. We don't go to court and initiate legal proceedings unless we hit problems to do with inheritance, but the issues shown on court records don't show particular animosity or unusual difficulty, but rather basic problems to do with fundamental aspects of marrying your children! There is evidence that dynastic incest certainly was practiced and praised even (see the Dura Europos as an example, 33CE) which confirms that at least the nobles, and some clergy, followed this religious teaching. Later on (post Sasanian) the term changed to mean only marriage between cousins. This was, as I said earlier, relatively common, both at the time and much later. The change in term meaning appears to show xwēdōdah between closer relatives had become either rare or taboo.

It was practised for a while within Zoroastrianism. James Darmester, writing in the 19th century, wrote that it was very rare for the Parsi to marry outside of the family, as it was considered to be a marriage made in heaven. Close biological relations (eg brother/sister, child/parent) were taboo and illegal by then, but cousin-cousin marriages were common, as they are in many parts of the world today.

I don't know about Crusader kings 2. It would depend on the time period, but there is little evidence to show how common incest was within ordinary Zoroastrian families. It goes against evolutionary behaviour and most cultures, but that doesn't mean much at all- and even in cuitures where dynastic incest was practiced and accepted (eg Ancient Egyptian culture) we have no evidence it was common among ordinary people, so anything I post about that would be speculation, although, in my opinion, it was unlikely, and judging from the little evidence and the taboo which developed, I'd suggest it was uncommon- although not unheard of at all- for it to occur amongst lower class families.