This does not have anything to do with occults, I've seen this in my normal human sims. As long as they are not blood-related, and you have an option for romantic interactions, they can get married to each other. A while back I was playing a household with 3 teens, two boys, and a girl, and none of them were related to each other. While they were teens I did not get them involved with each other, but after they became young adults, two of them became romantically involved and did get married to each other. They had the super sibling relationship.