Hi sosick, some possiblities. It mentions that Adam knew his wife Eve and she conceived. It mentions her first and second child (Cain and Abel), but time passed before Cain murdered his brother, so within that time frame, we do not know how many years passed and whether Eve conceived more children within that time period.

Also, the memory of what Cain had done, being the first murder, his hiding may have been the fear of (future?) vengeance from siblings who came later? When you consider either ones that were either alive at the time but younger, or ones that would be born later, the fear of them finding this out and taking revenge on him would obviously hang over his head. Particularly as these people had VERY long lives and "an eye for an eye, a tooth for a tooth" type justice was very much the way of things then. And something like this would be considered worthy of severe punishment, so he would have that mark on him all his life.