Although the Bible doesn't say, Joseph Smith provided the answer in "The Inspired Version" of the Bible, which was his very own "translation". Here's what it says:
And it came to pass, that Cain took one of his brother's daughters to wife, and they loved Satan more than God. – Gen.5:13
So according to Joseph Smith (and he ought to know!) Cain married one of his nieces. He doesn't say, though, which of Cain's brothers was Mrs. Cain's father.
Jonathan Kirsch suggests in The Harlot by the Side of the Road that the "play" between Isaac and Ishmael may have been of a sexual nature, noting that the same word is used to describe the behaviour of Ishmael and Isaac as is used in 26:8 to describe Isaac's fondling of Rebekah. 21:9