Was Ishmael, Abraham's eldest son a legitimate son?
Judeo-Christian scholars argue that since Ishmael was born to a concubine (Hajar), he is not a legitimate son.
However, according to Judaism, the gifting of concubines from barren wives to their husbands in order to produce offspring was a common, valid and acceptable occurrence, and the child produced by the concubine would be claimed by the wife of the father, enjoying all rights as if an own child, including inheritance.