It's all relative. Thoughts and opinions on a family matter.

I must admit this always puzzled me somewhat. It is evident that the reason it became taboo for close relations to marry in some societies was that over time it had been observed that the children resulting from such unions often had some sort of genetic weakness. This of course is due to recessive genes coming to the fore but when the science behind this wasn't known, people felt that it must be the result of the curse of the gods. For this to have been the case in Thedas, there would have had to have been enough instances of this in the past for the taboo to establish itself, although since the Maker seems singularly uninterested in the sexual activity of his worshipers, it could hardly have been regarded as a religious taboo.

Since Orlesians would have no objection to them being gay lovers and they certainly couldn't have children, it seems odd that it should have caused such scandal, bearing in mind they were cousins and not siblings. As others have pointed out, there are many royal families from the Egyptians onwards where marrying close relatives was the norm and whilst Christian societies might have baulked at siblings, there certainly seemed many instances of first or second cousins being paired off, probably in order to keep wealth and power in the family. So it is hard to see where this particular cultural objection on the part of the Orlesians does come from.