Yeah, brothers and sisters would have had to marry at first.

The anti-incest law God created wasn't a 'universal' law, per se. It was a law that was meant to be followed AFTER it was given. Likely, it was meant to protect people against the accumulation of harmful mutations or something along those lines.

"The task force finds that...the unborn child is a whole human being from the moment of fertilization, that all abortions terminate the life of a human being, and that the unborn child is a separate human patient under the care of modern medicine."