Genesis 28:9
Then went Esau unto Ishmael, and took unto the wives which he had Mahalath the daughter of Ishmael Abraham’s son, the sister of Nebajoth, to be his wife. Then went Esau unto Ishmael, and took unto the wives which he had Mahalath the daughter of Ishmael Abraham’s son, the sister of Nebajoth, to be his wife.
Esau took an additional wife besides the two named earlier, Mahalath the daughter of Ishmael, Abraham's son, and the sister of Nebajoth. Mahalath was also known as Basemath, the difference in names being explained by the common practice of people having more than one name.
The marriage took place after Esau's Hittite wives became a grief of mind to Isaac and Rebekah and after Isaac and Rebekah sent Jacob to Padan-aram to take a wife from his own relatives. It followed Jacob obtaining by deception the blessing Isaac intended for Esau, which forced Jacob to separate from his mother and flee from Esau, with his mother never seeing him again. This marriage does not undo the family division or the choices that brought it about.