Jeremy and Elena are cousins. Her biological father was John Gilbert, brother of Jeremy's dad. And Mystic Falls isn't in Arkansas or West Virginia.
You've got that backward. Cousin marriage is illegal in both Arkansas and West Virginia. It is perfectly permissible in regular Virginia, where Mystic Falls is located.
That only took a five second google search to find out.
And here I thought we were just joking around. Allow me to inaugurate a new shipperdom: Jelena!