What was the relationship between Ishmael and Esau?
Esau married Mahalath, the daughter of Ishmael, to please his father Isaac. Genesis 28:8-9
What the Bible says
Genesis 28:9
He went to Ismael, and took to wife, besides them he had before, Maheleth the daughter of Ismael, Abraham’s son, the sister of Nabajoth.
Douay-Rheims Bible (Challoner revision), public domain.