I heard this lesson long ago from a series called The stories of the prophets.
How did the children of Adam and Eve breed? Was it incest?
Adam and Eve had their own set of rules (arabic: Sharia) specific to their circumstance.
Eve gave birth to twins everytime she was pregnant, one son and one daughter.
And the ruling was that each pair that was born together was not allowed to get married.
By the way, as far as I know the only problem for a brother and a sister > to have a child is mutations in DNA.
If Adam and Eve had original DNA without mutations
then their children could make healthy grandchildren.
I don't think that is right. Biology is more complicated than that I think.
I personally think, although interesting to know, it doesn't really matter. It is speculation at the end of the day.