In a nation where government is required to stay out of the religion business, there is no legal way for it to establish "Biblical norms" for society.
However, there are perfectly good scientific reasons to avoid close relative mating. If, as the story indicates, they cannot have children, then it comes down to two consenting adults. So is it the responsibility of government to make us good and to keep us from sinning? Or is government's functions those asserted by the Founders of our nation:
...all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men...
I'd say the latter. So if two people want to do something together that is personally abhorrent to me, but harms no one else, I can't find a compelling reason to outlaw it.