I see two obvious underlying questions.  Isn’t it forbidden for brothers and sisters to get married and have children?  And is it even possible for two people to populate an entire planet?  Ok to answer the first question we have to look at genetics.  The average adult has around 36 trillion cells of which many carry chromosomes, half from your mother the rest from your father.  These contain about 25,000 genes which store your traits like height, hair and eye color.  Sometimes, by no fault of the parent, a gene may become abnormal.  These genetic abnormalities, which are passed by the parents to the children, may cause certain conditions like Down syndrome and Cystic fibrosis.  Parents can pass these genes along and not even have the condition themselves.  The risk of parents who both have similar genetic abnormalities, to result in a condition for their child, is usually small.  But, with sibling parents this increases dramatically because both have inherited the exact same genetic abnormality.  Many farmers have witnessed this first hand in feral cats (barn cats).  The first few litters are usually fine.  But, as time goes on and they begin inbreeding, the following generations will have greater instances of still birth and abnormalities.

So how does that relate to us?  Well, God created Adam and Eve with perfect bodies.  He then places the couple in the perfect environment, the garden of Eden.  He blesses them and tells them to be fruitful and multiply to replenish the earth.  Unfortunately, they disobey God, which allows sin to enter the world.  These perfect bodies and this perfect world become corrupt and imperfect.  So, slowly over time, genetic abnormalities begin to appear in those 25,000 genes.  It may take several generations, but eventually conditions start to appear in their children.  Adam and Eve’s children were free to marry without concern, because their genes contained very few genetic abnormalities.  In fact, this didn’t become a major concern until about 3400 years ago.  The ravages of sin caused genetic defects to occur so often, that God restricted who we should have children with.  This is found in the books of Leviticus and Deuteronomy.  So yes, thousands of years ago siblings married and had children, but currently it is forbidden.

As for the question of how could two people populate a planet, let’s do some math.  In the past people had large families sometimes more than 13 children.  So, let’s say a couple has 6 children.  Then each of those children had 6 children.  After 5 generations you would have over 7,000 people.  So even including variations in size of family and premature death, the math would suggest that two people, given enough time could certainly populate a planet.

Lastly, something you may have noticed with the passing of time, is that sin seems to be making everything worse.  There seems to be more disease, more crime, more hate and war could happen at any moment.  All of this can overwhelm any of us.  It can even cause a sense of hopelessness.  Just know that God is now and has always been in control.  He has created everything and he knows everything that has happened and will happen.  That is why he sent the answer, his son Jesus, who’s death paid the penalty for all of our sin.  Now, this redemption will not remove sin from your life, it will not make all of your troubles go away.  But it will restore your hope, knowing that this is not your final destination.  There is life after death, free of these troubles.  God wants you to be his child, it costs you nothing, the price has already been paid in full.  You just have to choose.  The choice is yours.