Technically speaking, the choices weren't merely sisters. Adam and Eve were having kids for 500+ years. Cain could have married his niece. We have no idea how old he was by then. For all we know Abel's daughters could have been of age. And even if the only choices were Adam and Eve's kids, Abel's kids or Cain's kids, as far as who to mate with, give it 200 years of child bearing, it could well be that their kids were coming of age to be parents and/or grandparents. Guaranteed that many people would have to start spreading out for hunting and gathering purposes fairly quickly.
Take it to the UK. If a couple had 500+ kids in that many years, and their kids lived about as long as the parents, exactly how long do you think it would take to populated the UK with all those descendants doing what comes naturally -- having their own kids?
After all, Adam died just a few decades before Noah was born.