It appears that your tutor has not read the biblical account very carefully. Genesis 4:25 states very clearly that after Abel's death, Adam and Eve had another son, named Seth. The Bible goes on to say in Genesis 5:4 that Adam fathered "sons and daughters" after Seth was born.
We can infer from the account that Cain's wife, mentioned in Genesis 4:17, was a daughter of Adam and Eve. In those early days of the human race, Adam and Eve's sons married their sisters, as there were no other human beings.
Your question actually touches on a broader topic—the debate in some schools over whether to allow the teaching of "intelligent design" as an alternative to evolution. What is sad is the degree to which some educators go to imply that evolution is the only "intelligent alternative" in the debate! It most certainly is not!
Actually, the "intelligent design" explanation is the only one that can satisfactorily explain how any life ever appeared or how this planet even appeared. So stand your ground and don't let yourself be intimidated or be made to feel that what you believe is somehow "uneducated." Many intelligent and thoughtful people do not agree with the basic premise of evolution.
See the article, "Darwinism in the Classroom," which appeared in the Fall 2003 Vertical Thought, for a more complete treatment of this subject.