Wherefore she said unto Abraham, cast out this bondwoman and
her son
Hagar, Sarah's handmaid and bondservant, and her son Ishmael; by this it appears that Hagar was concerned in this affair, and set her son on to mock Isaac, at least she encouraged him in it, buoying: him up with his being the firstborn, and having a right to the inheritance; wherefore Sarah saw plainly that there would be no peace nor comfort for her and her son, unless Hagar and her son were turned out of doors, for which she moves Abraham; and this not merely in a passion, but by divine direction and influence, as is evident from God's approbation of it: for the son of this bondwoman shall not be heir with my son, [even]
with Isaac;
which he would seem to be, if continued, and would think himself so, and there would be continual bickerings about it; wherefore, to put it out of all doubt who was heir, she desires that he and his mother both might be cast out of the house, which would be a clear determination of this matter. Sarah may seem to take upon her too much, to be so peremptory, as to declare who should, and who should not be heir, which more properly belonged to Abraham, whom she called her lord, ( Genesis 18:12 ) ; but what will sufficiently free her from any charge of this kind is the revelation of the divine will, and the promise of God that so it should be; namely, that the covenant God had so often renewed with Abraham should be established with Isaac, and not with Ishmael, ( Genesis 17:19-21 ) . Now what was the design of God, in guiding Sarah to make such a motion as this to Abraham, is taught us by the Apostle Paul, who makes these two women to be types and figures of the two covenants, and their sons of those that are under them, see ( Galatians 4:22-31 ) .
The Brenton translation of the Septuagint is in the public domain.