Jamieson, Fausset and Brown on Romans 9:10-13

10-13. And not only this; but when Rebecca, &c.--It might be thought that there was a natural reason for preferring the child of Sarah, as being Abraham's true and first wife, both to the child of Hagar, Sarah's maid, and to the children of Keturah, his second wife. But there could be no such reason in the case of Rebecca, Isaac's only wife; for the choice of her son Jacob was the choice of one of two sons by the same mother and of the younger in preference to the elder, and before either of them was born, and consequently before either had done good or evil to be a ground of preference: and all to show that the sole ground of distinction lay in the unconditional choice of God--"not of works, but of Him that calleth."