She was 90 something.
And barren.
Miracle. Anyway irrelevant. At least to the fact that Ishmael was a part of Isaac's Covenant, otherwise I wasn't ever interested. Brought it up in passing only.
But you said "promised son through Isaac" I am pretty sure.
Which would be not Isaac.
I have found a promise that God made to Abraham, and also to Sarah, at Genesis 21:12 B where God plainly says that Abraham should listen to Sarah for in 'Isaac' thy seed ( singular offspring ) would be called.
God would bless Sarah according to Genesis 17:16, and at Genesis 17:19,21 the ' everlasting covenant ' ( contract ) would be with: Isaac.
Thus, the promised ' seed ' ( offspring ) of Genesis 3:15 would prove to be Jesus through Isaac.
Jesus will fulfil God's promise that ALL families of Earth will be blessed, and ALL nations of Earth will be blessed.
Blessed with the benefits of healing for earth's nations according to Revelation 22:2; Genesis 12:3; Genesis 22:18.