Does Cain still have descendants on the earth? According to Genesis 5:


  3 When Adam had lived 130 years, he had a son in his own likeness, in
  his own image; and he named him Seth. 4 After Seth was born, Adam
  lived 800 years and had other sons and daughters. 5 Altogether, Adam
  lived a total of 930 years, and then he died.
  
  6 When Seth had lived 105 years, he became the father[b] of Enosh. 7
  After he became the father of Enosh, Seth lived 807 years and had
  other sons and daughters. 8 Altogether, Seth lived a total of 912
  years, and then he died.


If we continue reading, we see that we are descendants of Seth as his genealogy leads to the birth of Noah. God destroyed other families and spared only the house of Noah (8 people), from whom the earth was repopulated.

Adam->Seth-> ... ->Methuselah->Lamech->Noah->(Noah's Sons)-> .... ->Us


  Genesis 7:23- Every living thing on the face of the earth was wiped
  out; people and animals and the creatures that move along the ground
  and the birds were wiped from the earth. Only Noah was left, and those
  with him in the ark


Cain's descendants and Seth's siblings' descendants were therefore wiped out.
Your answer seems correct. As God has never approved of being unequally yoked. Cain's descendants followed after Cain and dwelt in his city (Gen 4:17). God never wanted people to live clustered in cities, where sin would abound, and there are various examples of this throughout Genesis. God's people stayed in the wilderness, tilling the ground and doing the work God had commanded to them as a blessing, even though the ground was now cursed.  These were the people that began "calling on the name of the LORD." (Gen 4:25)

You could see the striking difference between the people that followed God and obeyed His commandments and those that did not.

From this point you can infer that Noah and his sons, being righteous in the eyes of God, would not mix with those who did evil in the eyes of God. If this is so, then Noah and his 3 sons would have wives that loved the LORD and were not of the lineage of Cain.

Of course it could also be that one or more of the wives were descendants of Cain that decided to follow in the way of the LORD. Nevertheless, the Bible offers no such insight on this matter. So the true answer is we do not know, but I'm definitely more inclined to say that they all were destroyed.