Since it is ordinary Catholic teaching that all men and women descend from Adam and Eve, Cain's wife likely was a descendant of Adam and Eve. If so, that means that Cain's wife was his sister. Adam and Eve had more children than those who are named in the Bible, as is recorded in Genesis 5:4
. For more information and greater discussion of the issues involved, please see the article linked below.
Adam, Eve, & Inbreeding
by Jimmy Akin