With whom did Abel and Seth marry? Adam and Eve have two sons, Abel and Cain.
Cain slew Abel because of jealousy.
Then Genesis 4, Cain got married, but to whom? 
Adam and eve were the first two persons in this world and that makes their sons the third and fourth person. 

With whom did Abel and later his younger brother Seth got married.
The question assumes a literal young-earth, literal creation and Adam and Eve as the literal first and only people interpretation of Scripture, so this answer comes from that view.

The common response from pretty much every Apologetic YEC organization is that they married their sisters. There were no other women in the world. And besides that, there were no laws forbidding marriage between brothers and sisters, but had there been; only God was in authority at the time, and he already knew.

As explained on  ChristianAnswers.Net


  ...Even though only these three males are mentioned by name, Adam and Eve
  had other children. In Genesis 5:4 a statement sums up the life of
  Adam and Eve—“And the days of Adam after he had fathered Seth were
  eight hundred years. And he fathered sons and daughters.” This does
  not say when they were born. Many could have been born in the 130
  years (Genesis 5:3) before Seth was born....


Also from biblestudy.org


  ANSWER: The debate over who became the wife of man's first two
  children has gone on for centuries. The short answer is that the
  brothers had to marry their sisters. Man was, at first, only two
  people, Adam and Eve. The couple's first three children were Cain,
  Abel and Seth: "After he begot Seth, were eight hundred years; and he
  had sons and daughters" (Genesis 5:4).
  
  How many sisters were potentially available for Cain and Abel to
  marry? The Bible does not reveal exactly how many children Adam and
  Eve produced. It is safe to assume, however, that since Adam lived 930
  years, he had PLENTY of time to take full advantage of God's command
  to procreate and fill the earth!


And the answer to the prohibition against Incest is consistent among apologetic groups.

For future research, I recommend a Bing or Google search on "Who did Cain marry".  You'll find that the answers that you find from those who hold the literal interpretation of Genesis, the answer is consistent.