In Genesis 20:10-12 10, this is made clear: And Abimelek asked Abraham, “What was your reason for doing this?” 11 Abraham replied, “I said to myself, ‘There is surely no fear of God in this place, and they will kill me because of my wife.’ 12 Besides, she really is my sister, the daughter of my father though not of my mother; and she became my wife.
In the Book of Jubilees, it is said that Seth married his sister, Azura. But I find no reference of this in the Bible, not for Caine or for Seth.
Genesis 4:17 notes: "17 And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son—Enoch." There is literature that suggests Cain did marry the oldest daughter of Adam and Eve.
I suspect you are correct. I just wish there was more in Genesis to discuss the daughters.
Rtm