Jamieson, Fausset and Brown on Hebrews 11:18

18. Of whom--rather as Greek "He (Abraham, not Isaac) TO whom it was said" [Alford]. Bengel supports English Version. So He 1:7 uses the same Greek preposition, "unto," for "in respect to," or "of." This verse gives a definition of the "only-begotten Son" (He 11:17).

in Isaac shall thy seed be called--(Ge 21:12). The posterity of Isaac alone shall be accounted as the seed of Abraham, which is the heir of the promises (Ro 9:7).