So she went to Abraham and told him, “You have to get rid of this slave woman and this son of hers! A slave woman's son is not going to be one of your heirs and inherit together with my son Isaac!”
“No, it's your wife Sarah who is going to have a son for you!” God replied. “You are to call him Isaac. I will keep my agreement with him and his descendants as an eternal agreement.
If the inheritance is derived from obedience to the law, it no longer comes from the promise. But God graciously gave it to Abraham by means of the promise.
They left us, but they didn't belong with us, for if they did, they would have remained with us. When they left they showed that none of them belonged with us.
Gilead's wife gave him sons, who when they grew up, drove Jephthah away, telling him, “You won't inherit anything from our father because you are another woman's son.”