How were Rebekah and Isaac related before being married?

According to the Book of Genesis, the patriarchal family line is a self-contained unit, originating solely in Terah, with no pathways to or from the outer world. Abraham, Isaac, Jacob and even Joseph were all descended on both sides from Terah.

Sarai was Abram's sister and therefore not only Isaac's mother but also his aunt, so that any further genetic relationship would be, in effect, doubled. Rebekah was the daughter of Bethuel, Isaac's own cousin through Nahor. Bethuel was also the son of Isaac's own cousin Milcah. Jacob, in turn married his cousins, Rachel and Leah. In this story, Rebekah and Isaac were closely related, as second cousins on both sides, separated by one generation through Nahor and Bethuel, or two generations through Haran, Milcah and Bethuel.