I’m sure this one is asked and discussed all the time but Search isn’t finding anything on it. Within the context of the Bible, how do Christians explain how humans could have propagated without either Cain or Able doing it with Eve? 
Not Eve, their sisters.
The issue of incest (or alternatives) has been mentioned in a few other threads, including
bible question 02-16-2003
How many people did God create? 11-27-2001
Where did Cains wife emerge from? 12-14-1999
Who had Cain and Abel’s babies? 10-10-1999
Genesis creation in bible doesn’t make sense 04-13-1999
And a couple others I’ve probably missed or that appeared in Great Debates.
And, of course, there are many Christians who are not biblical literalists who would hold that the story (with specific people and events named) is an expression of mythology, so that the question is not particularly relevant.
Genesis 5:3-4