John Calvin Commentary Genesis 21:10

"Wherefore she said unto Abraham, Cast out this handmaid and her son. For the son of this handmaid shall not be heir with my son, even with Isaac." — Genesis 21:10 (ASV)

Cast out this bondwoman. Not only is Sarah exasperated with the transgressor, but she seems to act more domineeringly towards her husband than befitted a modest wife. Peter shows that when, on a previous occasion, she called Abraham lord, she did not do so insincerely, since he presents her as an example of voluntary submission to pious and chaste wives (1 Peter 3:6).

But now, she not only usurps authority in the household by calling her husband to order, but commands him whom she ought to revere to be obedient to her will. Here, although I do not deny that Sarah, being moved by her emotions as a woman, overstepped the bounds of moderation, yet I do not doubt that both her tongue and mind were governed by a secret impulse of the Spirit, and that this entire matter was directed by God’s providence.

Undoubtedly, she was the instrument of a great and momentous judgment. And Paul cites this expression, not as a futile reproach, which an enraged woman had uttered, but as a heavenly oracle. But although she acts in a capacity beyond that of a private woman, she still does not deprive her husband of his authority, but makes him the lawful director of the expulsion.