This has to be one of the most frequently asked questions we get here
at Blue Letter Bible. More likely than not, Cain married one of his
unnamed sisters (or perhaps even a niece). Adam and Eve likely had
many children by the time of Cain's exodus to the land of Nod and
whether Cain had already married or acquired his wife post-exile, it
would not require any interpretive gymnastics to understand his wife
as having been a close relative. Just because no sisters are directly
indicated does not mean that Adam and Eve had no daughters at the time
of Abel's murder. A child of
the patriarchs was not usually mentioned unless he fulfilled one of two
narrative functions: he was integral to one of the stories within the
Jewish mythos or he was to become the progenitor of a nation. If either
of those two functions were not fulfilled, a child's existence was
rarely even indicated (especially if that child was female); therefore,
it is really no great shock to find that Cain married a woman who earned
no previous mention in Scripture.