This is an issue which can't be "sent back to the states". That just won't work.
It must either be affirmed (i.e., a refusal to hear the case), or reversed.
I've been desperately searching the net for an old site that detailed the process that's followed when State Laws conflict re: First cousins can legally marry in 18 States of the Union, Alabama being one of them.
So Billie Jean and Bobbi Joe get hitched in 'Bama, then their job transfers them to Idaho. What then?
IIRC In spite of the fact that Idaho does not allow first cousin hookups, they will recognize the Alabama marriage as legal and afford the couple all rights that come with the it.
So in theory, it could be left to the States.