Brother With Sister and Sister With Brother?

According to the Bible, Adam and Eve had 2-sons who were Cain and Abel. Cain killed Abel leaving only Cain. Genesis 4:16 & 17 says Cain settled in the land of Nod and his wife became pregnant. If Adam, Eve and Cain were the only 3-people on earth at this time, where did Cain's wife come from? Are we to ume Cain's wife was his sister?