Genesis 4:17 And Cain knew his wife (one of his sisters) and she conceived,and bare Enoch

Adam and Eve had sons and daughters. Cain married his sister. ( Genesis 4:17 a.r.w. Genesis 5:3-4),

We know that Cain lived several hundreds of years before The Flood of Noah,

The flood of Noah happened when Noah was 600 years old.

Before the flood of Noah, the human blood stream was not as weak as it is today. People lived longer. Adam lived to be 930 years ( see Genesis 5:1- 5) , Lamech lived to be 777 years old  ( See Genesis 5:31) and at the time of the flood Noah was 600 years old.( Genesis 7:6).

WE know that Moses was born several years after the flood of Noah.

However, in the days of Moses, i.e. sometime after the flood of Noah, when men and women began to multiply again on the face of the earth, we see that at The Time when GOD gave The Law to The Prophet Moses to give to Israel, we see that incest was condemned.( see Leviticus 18:1-19 etc).

All Bible Believers agree that the flood of Noah happened about 1656 years after Adam and Eve were created in The Garden of Eden.

The skeptic notion that Cain married his mother is wrong.

The skeptic's notion that Cain may have married a gorilla or a chimpanzee or some ape is also wrong.

GOD made all nations from one blood.( from Adam and Eve).

The Holy Bible says in Acts 17:26 that

"...  And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;.."( Acts 17:26).

The Holy Bible is a sealed Book to unbelievers.

In Isaiah 29:11 we read :

"...And the vision of all is become unto you as the words of a book that is sealed, which men deliver to one that is learned, saying, Read this, I pray thee: and he saith, I cannot; for it is sealed:..".

Unbelievers, Skeptics Mockers and Scoffers always mis-interpret The Holy Scriptures.

No Scripture is of Private Interpretation.

"...  Knowing this first, that no prophecy of the scripture is of any private interpretation...". ( 2 Peter 1:20).

The Holy Bible interprets itself. Scripture is interprets by Scripture.

Adam and Eve had sons and daughters,

"...3 And Adam lived an hundred and thirty years, and begat a son in his own likeness, and after his image; and called his name Seth:

4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:..". ( Genesis 5:3-4).

Abraham ( Abram) married his cousin Sarah ( Sarai).

"... Say, I pray thee, thou art my sister: that it may be well with me for thy sake; and my soul shall live because of thee.".( See Genesis 12:13).

Abraham said that Sarah, told Sarah his wife to say that he Abraham was her brother.

Adam and Eve had sons and daughters and so Cain married one of his sisters.

"And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.". ( See Genesis 4:17).