“The Bible without any doubt teaches that God chose to start
humanity off from only two people. This means that in the first few
generations, marriage had to be between extremely closely related
people, including at least one brother–sister union. The Bible says
that Adam and Eve had daughters as well. Cain could have married
his sister, or niece.
The biological problems caused by such unions today come from
a progressive accumulation of genetic defects since the Fall. An
originally perfect population would not have had any such problem.
The Bible teaches that even Abraham, living a long time after
creation, was still able to marry his half-sister, Sarah, without
any hint of biological problems in the offspring (Isaac). In doing
so he was not breaking God’s Law. The Law of Moses forbidding
intermarriage between close relatives was not given until centuries
after Abraham.” …”