That's right, they're step-brothers. Abraham was their father, and they've argued since then about who inherits the Holy Land.
I was once talking to a Kuwaiti about this (our eldest lives there) and he said that in the story about God ordering Abraham to sacrifice his son Genesis 22 :1 to 2 their Bible said it was Ishmael (Hagar's son) not Isaac (Sarah's son).
Ishmael was in fact Abraham's firstborn.
Retiring and living frugally in money from downsizing after years of stress

