An objection I’ve heard to the creation account in Genesis is that the children of Adam and Eve would have had no one to marry. The question, “Where did Cain get his wife?” is a common one.  

However, God did not forbid marrying one’s sister until the Law of Moses (Lev 18:9). Over two millennia after the creation, Abram married his half-sister, Sarai (Gen 12:19; 20:2). While that was probably unusual at that time, it was not unheard of, and it was not forbidden by God. 

Here is what Answers in Genesis says: 

Of course, one question that naturally follows is, whom did Adam and Eve’s children marry—where did their first son, Cain, get his wife? Well, if God created only two people from whom the entire human race would come, then Cain had to marry either a sister or a niece. Up until 2500 years after creation, it was not uncommon or sinful for close relatives to marry. It was not until the time of Moses, in Leviticus 18:6, that God prohibited marriage of close relatives. Today such marriages can produce children with genetic defects, since both parents are more likely to be carriers of the same defective genes. But the accumulation of defective genes is a gradual process and would not have been problematic for our earliest ancestors (see here).

While not all Evangelicals are Dispensationalists, all recognize that God has changed His laws throughout history. Restricting degrees of kinship in marriage is but one example of changing laws. During the Law of Moses, eating pork or many other foods was sinful. Today, all foods are lawful. Offering animal sacrifices was required during the Law of Moses. Today we do not offer animal sacrifices.  

Some Jewish speculation suggested that Cain and Abel each had twin sisters. They each married the other’s twin sister. Of course, the Bible does not say if Cain or Abel had twin sisters.  

The Biblical account of creation is accurate because it is part of God’s Word. Since God is true, His Word is true as well. Skeptics have long attacked the Bible. But the Word of God abides forever (Isa 40:8; 1 Pet 1:23, 25).  

Keep grace in focus and you will remain committed to the inerrancy of Scripture.