Where Did Cain Find His Wife?

Where did Cain find his wife (Genesis 4:17)?

Adam had “sons and daughters” (verse 5:4), so Cain’s wife was probably a sister (though some think God may have created other human beings besides Adam and Eve). Marriages between close relatives were at first unavoidable if the whole human race came from a single pair. Only later was marriage between siblings prohibited (Leviticus 18:6-18).

–The Quest Study Bible