Sorry to raise the Ewww gross factor...
But a lot of ancient royal dynasties had a lot of inbreeding, mostly to keep power "in the family". Brother-sister was not unheard of in ancient Egypt and I think first cousins was not uncommon at all in Medieval times.
Now in Elemental it seems at least possible that you may be marrying people to each other for certain genetic traits, esp magic ability. I am just wondering if there are going to be any rules in place to keep you from marrying siblings to each other, will there be negative consequences for doing so (I think birth defects and crap like that would be a bad idea, but possibly a chance of having squib offspring (for non-Harry Potter fans that is two parents with magic ability having a child with no magic ability).
Of course, they may very well jsut make the game so you have to marry of kids in exchanges with other factions to avoid the question alltogether. And I apologize to those of you with decent sensibilites for raising the subject.