Why did Demeter have a relationship with Zeus if they were brother and sister?

It is a common theme in Greek mythology, and other ancient myths as well, for siblings to marry one another and have children together. This could partially be due to the fact that the Greek Gods, for the most part, were all from the same family and there were no other gods to marry. It could also be a reflection of the ancient idea that it was the bloodline of rulers that made them important and the practice of some royal families to closely intermarried in order to keep their bloodline pure.