[quote="ChibiViolet, post:19, topic:112602"]
The Bible does imply it though. I guess that means they would have had to have been Cain's older siblings?
Actually, I found another possible explanation that's much more likely from Christian Forums:
Adam and Eve had other children, probably beginning right after the birth of Cain and Abel. Seth was born really to replace Abel:
And Adam knew his wife again, and she bore a son and named him Seth, “For God has appointed another seed for me instead of Abel, whom Cain killed"-Gen 4:25
Seth's birth may have occurred soon after the death of Abel. If so there was lots of time for other children to have already been born between the birth of Cain and Abel and Seth, and to become a sizable population in the area. Note the time of Seth's birth in regard to Adam's age:
3 And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. 4 After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters. Gen 5:3-4
Note the semi-colon highlighted in blue/bolded. This indicates a separate or additional thought not completely within the lineal context of the rest of the sentence. In other words it suggests that Adam at some time in his life had other children. It is probable that Adam and Eve were barely out of their teens when Cain and Abel were born. This leaves one hundred and ten or so years for those other children to grow into large numbers before the death of Abel/birth of Seth.
Another (possible) indication of a growing human population is the "process of time" phrase. It is possible that God wanted to begin the sacrificial system at that very time because of an increasing sinful population. Abel was actually in the position of High Priest, and would bring a suitable sacrifice in the form of an animal, which would be sacrificed for the sins of the people (Abel himself is called "righteous" elsewhere). Note that after his death men did not "call upon the name of the Lord" until the time of Enosh, son of Seth. At which time there would have been quite a large population.