Why is Cain afraid when he is driven away from God? When God punishes Cain, he replies:


  Gen 4:14 Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth,
  and whoever finds me will kill me. (NIV)


All are his brothers and sisters? And he is the elder brother of all of them? So who is Cain fearing?
Cain was likely afraid of one of his brothers or sisters taking revenge by killing him.  Yes, he was the older brother, but he was also guilty of murder.  If Cain was willing to kill one of his brothers (Abel), then it is reasonable to think that someone else was willing to do the same.  And, as you mentioned, there was no one else around to fear, so Cain did fear that his siblings would take revenge for Abel's sake and take his life.