What was the relationship between Ishmael and Isaac?
Ishmael and Isaac were sons of Abraham, by different mothers. When Abraham died, they buried him together. Genesis 21:9; Genesis 25:9
What the Bible says
Genesis 21:9
And when Sara had seen the son of Agar the Egyptian playing with Isaac her son, she said to Abraham:
Genesis 25:9
And Isaac and Ismael his sons buried him in the double cave, which was situated in the field of Ephron the son of Seor the Hethite, over against Mambre;
Douay-Rheims Bible (Challoner revision), public domain.