If you have not read Stories Versus Truth, you may want to read that before reading this.

Question:

You may get a question that presupposes evolution and millions of years like this, "In the book of Genesis 4:17, And Cain knew his wife---where did she come from? There is no women to be Cain's wife."  Here is another example of the same reasoning, "If God used Adam's rib to create Eve, wouldn't that make her essentially a twin of Adams? Wouldn't that mean that their sex was incest? Thus their offspring should be deformed, no?"  Sometimes these questions come from a follower of Christ who is honestly searching and sometimes from a scoffer who isn't thinking clearly.

Answer:  

Keep in mind that disobedience to God started the degradation of everything.  This is where pain and suffering originated, but the degradation just gets worse over time.  It didn't happen all at once.  This degradation also affected the information in the cells of every living thng.  At first, there was no degradation.  It has taken 6,000 years for the information to degrade to the point where it is today. 

Cain took his sister--Adam and Eve's daughter--as his wife.  The human genome was still fully intact at that time, so there was no restriction on marrying one's sister.  As time passed, the genome was damaged.  I'll explain that a bit below, just in case you may be curious. As time passed, a restriction was placed on marrying siblings, but it was OK to marry some close relatives without fear of birth defects.  Abraham married his half-sister.  When the Law was given, many years later, God restricted marrying of close relatives.  
 
This part gets a bit more technical and you may or may not be interested.  The closer that two people are related, the more likely the chance of sharing genes for recessive traits that cause birth defects if they match up.  So, when a baby is conceived, a single cell from the mother joins a single cell from the father and the two cells join to form a new human being.  If the two are too closely related and have matching defects in the genes, then there will be a defect of some sort.  
 
Adam and Eve and their children had perfect or near-perfect genomes.  Abraham, not so perfect.  Moses, even less perfect.  So, there was not restriction against Cain marrying his sister.  The fact that things are different now than they were then has brought some Atheists to use this as an argument against the validity of the Biblical account, but Atheists can never understand the Bible, since understanding comes from the Holy Spirit by revelation.  
 
Genes are damaged by things like x-rays, cosmic rays, and such.  Think of the information in the cells that join as two libraries of books that are joining to organize their information into a new library.  Over the years of life, the mother and father are made up of many libraries (the genomes in cells) and those libraries are copies of other libraries through their lifetimes.  During those lifetimes, cosmic rays and other factors have been shooting at those libraries like crazy people with pistols.  When a bullet hits a book, it knocks out parts of words.  The cells are really good at fixing these broken words, but some get through.  No information is ever added by this process.  Information is always lost.  The mother and father had parents who also lost some information.  Their parents had parents, and since the beginning of time humanity has lost a bit of information over the generations.  We have no idea what kind of man and woman Adam and Eve might have been, especially before the fall.  Even now, the way that the human body is created, even in its fallen condition, is amazing beyond our wildest imagination and we have only a very limited understanding of how it works at present.  
 
We have witnessed that the destruction of the genome is ongoing over time.  The Second Law of Thermodynamics is called a law of science because it has been thoroughly tested and no exception to it has ever been found.  Things deteriorate to a lower entropy over time.  That means that there is less information and organization over time.  You can see this in a house that is not maintained.  It also happens with the fantastic amount of information that is stored in the genetic code of each person.  No information is ever created.  Information is continually lost.  God created our cells with the amazing ability to fix broken parts of that genetic code, but, once in a while, an error makes it past that quality control system.

This would also all apply to the question about Eve being formed from Adam's rib.