The law against incest was not given until later in Leviticus 18:6-30...up until that point it was not considered a sin...how do you think Cain got a wife, if not from one of his sisters?
What the Bible DOES make clear however is that there were
other people “in the lands to the east”, and that these strangers
presented a credible threat to Cain. Further, the Bible only refers to Cain as being subject to being a “wanderer”;
however not for the rest of his life. Based on the biblical time line (below), it would strongly suggest that Cain's wife came AFTER his time of wandering had come to and end, and would therefore not be his sister (and instead be one of those "other people" from the lands to the east):
a) God drove out Cain as a wanderer (nomad),
but nothing mentioned about any other person.
b) "Other people" out in the world will want to kill Cain,
but nothing mentioned about any other person traveling with him.
c) God will enact vengeance upon any of these "other people" if they were to kill Cain. Says nothing about his brothers or cousins, or uncles, or what have you. Specifically refers to them as separate people.
d) Cain began life as a nomad by traveling to the east.
e) Cain had a son with his wife. This is the first mention of anyone other than Cain,
and comes at a point when it appears he has stopped wandering. According to the biblical tales, Cain didn’t wander his whole life; he actually developed roots and started his own culture and built the city of Enoch.
a) Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth;
b) and it shall come to pass, that every one that findeth me shall slay me.
c) And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.
d) And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.
e) And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. –
Genesis 4:14-17
Therefore it is fairly demonstrable that the tales outline being driven out as a solitary nomad, eventually encountering other people in another land, then settling down, having children, and building city. In that precise order, it is fairly easy to discern that his wife was not his sister (or even his cousin; a possibility that was not mentioned).
OM