Plasma Productions

In answer of your three questions:

1. Believe it or not, Cain's wife was probably his sister.  We might look upon an act like this in disgust, but for Cain, there were no other alternatives because his sisters were literally the only women in the world, besides his mother.  What about the negative effects of incest like birth defects?  These occur because of negative mutations in our DNA.  Fortunately, most of these mutations are genetically recessive, which means they will will only produce defects in the offspring if both the mother and the father have the mutation in their DNA.  When two people from separate families marry, it is very unlikely that they will both have the same negative mutations.  If two people from the same family marry, it is more likely that they will have the same negative mutations, since their DNA is very similar.  This means that any children they have will likely have birth defects or other problems.  That's why, in the Hebrew Law, God outlawed sex between close family members.  It's considered a sexual sin. However, the Hebrew Law was not written until the time of Moses.  So at the time, it was lawful for Cain to marry his sister.  His children wouldn't have any birth defects or any other negative effects of incest, because Cain and his wife had no negative mutations in their DNA.  God created Adam and Eve with perfect DNA, and thus their children inherited that perfect DNA.  It wasn't until generations later that negative mutations started to arise.

2.  It's true that Cain and Abel were the third and fourth people on Earth.  But the Bible says in Genesis 5:4 that sometime in the eight hundred years Adam was alive, Adam had another son named Seth (the sixth person on Earth), AND "other sons and daughters".  Cain's wife was probably one of these daughters.

3.  If you're asking if there are other ancient texts besides the Bible which tell the story of Cain and Abel, then yes.  The Koran also tells the story of Cain and Abel, as do many folktales of various cultures (most of which were not published until the nineteenth century). See here for a comparison of different versions of the story of Cain and Abel: http://www.pitt.edu/~dash/cain.html#genesis

Remember that the Bible's account of the story is by far the oldest, and therefore it's the original story.  All other versions of the story are based on it.