Charles Ellicott Commentary Genesis 28:6

"Now Esau saw that Isaac had blessed Jacob and sent him away to Paddan-aram, to take him a wife from thence. And that as he blessed him he gave him a charge, saying, Thou shalt not take a wife of the daughters of Canaan." — Genesis 28:6 (ASV)

ESAU MARRIES A DAUGHTER OF ISHMAEL.

When Esau. The solemn transfer of the birthright to Jacob, and Isaac’s complete assent to it, must have caused Esau considerable grief. Evidently, it made him feel that he had greatly contributed to this outcome through his own illegitimate marriages.

Therefore, when he saw Jacob being sent away to obtain a wife, in accordance with the rule established by Abraham, Esau also decided to conform to this rule and married a daughter of Ishmael.

She is called Bashemath in Genesis 36:3, and is described in both places as the sister of Nebajoth, in order to show that just as Nebajoth, the firstborn (Genesis 25:13), was undoubtedly the son of Ishmael by his first wife (whom Hagar took for him out of the land of Egypt, Genesis 21:21), so Mahalath also shared in this precedence, and was thus not the daughter of any of Ishmael’s subsequent wives or of a concubine.