Matthew Poole and continuators on Genesis 28:9

9 Then went Esau unto Ishmael , and took unto the wives which he had, Mahalath ▪ the daughter of Ishmael Abrahams son, the sister of Nebajoth , to be his wife.

Either to his Person; or, rather to his Family called Ishmael by their Fathers name, as David is sometimes put for Davids posterity: for Ishmael seems to have been dead before this, from Gen. 25. 17. though that may possibly be a Prolepsis, and then this may be Ishmael himself.

Called also Bashemath, Gen. 36. 3. He thought by this means to ingratiate himself with his Father, and so to get another and a better blessing: but he takes no care to reconcile himself to God, nor observes his hand in the business. Besides he mends one fault by committing another and taking a third wife when he had one too many before, and her too he unwisely fetcheth out of that stock which was begotten to bondage and was utterly uncapable of the inheritance.

Ishmaels eldest son, Gen. 25. 13. who alone is here mentioned, either in the name of all the rest, whose Sister she is by consequence supposed to be; or because peradventure she and Nebajoth were Ishmaels children by the same Mother, and the rest by another.