A dear friend has asked for clarification about the first generations of the human race like, who did Cain marry, did he have many sisters, etc. I assume that Adam and Eve had many children over their long lives and that their children also had many children of their own. Am I correct in my assumption that we have no idea how old the brothers were before the murder?
Genesis 5:3-4 reveals that when Adam was 130 years old, he and Eve gave birth to Seth and during the 800 additional years he lived they had an unknown number of other sons and daughters as well. The Bible doesn’t say how old Adam and Eve were when Cain and Abel were born, nor does it say how old Cain and Abel were when Abel was murdered. Since there were no other offspring before Seth, then Cain most likely married one of his sisters .