Obviously Cain’s wife would have been his sister. Possibly a rebellious sister who had trouble with what God required.
Took after her mother.
Rather than being afraid of his siblings, or relatives, why did Cain fear death by others? Who were the others?
Might Cain be what's now called Cro-Magnon? Might his wife be either Cro-Magnon, or part of the other... a Neanderthal?
I don't think so. By that time, Adam and Eve had been long expelled from Eden. Don't assume no more people were created in the outside world just because the bible omits that information. It also doesn't mention the sister you assumed.
Wouldn’t that mean that we are all ancestors of a very incestuous family ?
If the immediate descendants of Adam and Eve lived for an extremely long time, then after a few hundred years, there were probably lots and lots of females for him to marry. Of course, that's quite a ways to wait for a sweetheart, I guess...
And if Adam and Eve were in a garden that intersected the spiritual and the earthly, then they could have been cast from it, and an angel could have been placed on guard to prevent re-entry to it. It would have contained the tree of life, a location of that tree reappearing at the end of the Revelation.
That casting from the Garden into the realm of the physical earth might well have brought them to a place where "the children of Adam" confronted children of men; 'male and female, He created them." And He kept NO plant from them.
Genesis 1: 27 So God created man in his own image, in the image of God he created him; male and female he created them. 28 God blessed them and said to them, "Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish of the sea and the birds of the air and over every living creature that moves on the ground." 29 Then God said, "I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.
Genesis 2: 7 the LORD God formed the man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being. 8 Now the LORD God had planted a garden in the east, in Eden; and there he put the man he had formed. 9 And the LORD God made all kinds of trees grow out of the ground--trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil. .....snip....15 The LORD God took the man and put him in the Garden of Eden to work it and take care of it. 16 And the LORD God commanded the man, "You are free to eat from any tree in the garden; 17 but you must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die."
This is another means of accounting for Cain's wife and for a distinction between those who were from Adam and some who were not.
This understanding, too, has its difficulties, but it is an alternative, that is, that the man and woman placed in that garden were unique and separate from the others.