Read Romans 9:7 Using Other Translations
Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
and not all are children of Abraham because they are his offspring, but "Through Isaac shall your offspring be named."
Being descendants of Abraham doesn’t make them truly Abraham’s children. For the Scriptures say, “Isaac is the son through whom your descendants will be counted,” though Abraham had other children, too.