You didn't answer the question, but you are correct as God did not forbid certain categories of incest until the giving of the 10 Commandments.
I think I did answer it. People people shagged their sisters regularly, back then. Maybe that's the wrong answer. What's the right answer? Is this a quiz?
It's not a quiz.
When the human race began and the population was small, people were immediate family obsessed.
This led to people treating each other with contempt and violence which resulted in God selecting a man whose family was immediate family and society oriented.
That person was Noach.
The experiment did not succeed as Nimrod was obsessed with being the supreme emperor of the world and was worse than immediate family obsessed; Nimrod was self obsessed.
God therefore dispersed the families of the Tower generation in order to at least refocus on family as opposed to self.
When God gave the Commandments, he wanted the Children of Israel, who were individual Tribe obsessed to be Children of Israel obsessed.
The experiment failed in part because, other than the kingship of Solomon, no nation ever became society obsessed.