How do you know he had no sister? Do you expect the Bible to list EVERY descendent of Adam and Eve? Or did they only have three sons?
We are not told how old Cain was when he killed his twin. Had to be at least at the age of accountability. The reading is that Cain was forced away from home, the ground was cursed to him. There is no mention of taking his sister with him, yet, Cain did find a mate in the land of Nod.
Given what we are told in Genesis 1:26 and the amount of time that passed until the formation of the Adam there would have been quite a population of hunter gathers. That is exactly what the history books describe, time before agriculture. Although once word gets out that the Bible also describes a time of hunter/gathers before agriculture they will most likely have to rewrite history.