Ishmael was an in-law father of his nephew Esau, and Esau was the twin brother of Jacob. Esau’s and Jacob’s common grandparent with Ishmael’s children (including Esau’s wife Machalat) was Abraham, whose brother Nahor married his niece (Haran’s daughter) Milcah (whom was the mother of Rebecca’s father, Bethuel). Abraham, meanwhile, married his (and Haran’s and Terah’s) sister Sarah.
So, think about this: Isaac and Rebecca were primarily once-removed first cousins, and Isaac was also a nephew to his father (Abraham) and to his mother (Sarah). He was a double nephew to Nahor and Haran, and an in-law nephew to his first cousin Milcah. He also was in-law first cousin to his uncle Nahor, and a direct and once-removed first cousin to Bethuel (which also made him a second cousin to Rebecca).
Keep in mind that this made both Isaac and Rebecca twice-removed first cousins to Jacob and Esau…
And I think that ought to humble us Jews and the Arabs a little more. We all are literally descended from a sibling couple (as Abraham and Sarah were children of Terah through different mothers), an avuncular (uncle-niece) couple, a multiethnic couple (It can be assumed that Rebecca’s unnamed mother was Aramean or Syrian, as Bethuel was Hebrew and Numbers describes Jacob as “‘a wandering Aramean’”), and a couple whom were once-removed cousins whom were related in other ways.