I guess there are 2 possibilities:
- God created a few more people (most unlikely)
- They had to get it on with each other (most likely).
Yes, that is detestable to us now, but back then it was fine! There was only one law, one rule: “Don’t eat the fruit from the forbidden tree”. And keep in mind that the gene pool was fresh and clean. There was no chance of mutation or genetic muck ups, so it was perfectly acceptable and neccessary to populate the earth.
So time went on and the population grew, and generations followed. And as the population increased it was no longer neccessary for brother and sister to have sex, there was a large quantity of cousins to choose from. More time passed and then came the time of Noah. The population of the earth gets wiped out and there is about 8 people on the boat; Noah, his wife, two or three daughters and their husbands. The in-breeding had to kick in again to repopulate the earth. A couple of thousand years passed, Gods laws were decreed, in crept disease and the genetic code began to degrade. Now everyone knows that its a bad idea.