Who did Adam and Eve’s sons marry? There is only one answer to that – they must have married their sisters. In those days, the human gene pool, perfect before the advent of sin, was still almost perfect. An incestuous sexual relationship nowadays would produce offspring quite likely to inherit a pair of adverse recessive genes with serious consequences to the child’s health. The likelihood of that occurring just after the Creation was very much smaller.
There must have been many such close marriages following the time of Noah and the flood, and that was still true of the marriages between the Hebrews in Old Testament times. By then, the hazards of the deteriorating gene pool and close marriages were becoming apparent, and God instructed His people, through Moses, to avoid specified close sexual relationships and other sexual activity which is an abomination to God. (For details, see the Old Testament book of Leviticus, chapter 18.) Certain incestuous relationships are currently forbidden by English law.