It is often asked where Cain got his wife. Genesis, chapter 5, gives the written account of Adam's lineage. From that chapter, we learn that Adam had sons and daughters; also, that he lived to the age of 930 years. No doubt Cain married one of those numerous daughters or granddaughters of Adam.
In other words, Cain married either his own sister or niece. In the dawn of history, such marriages would have been necessary and would not have been accompanied by the dangers which may result today. God later established laws to prohibit marriage within close degrees of kinship (Leviticus 20:17-20).
Intermarriage of closely related persons such as brothers and sisters today, according to doctors, would result in serious physical deterioration of the race.