While they are some well intended Christians who will try and make it seem as if God in the New Testament is kinder and gentler than God in the Old Testament, today's reading reminds us the God of both testaments is one.
Despite all that Sarai did, today we hear God proclaim her Sarah, marking her role in the Covenant with Abram who will be Abraham. God did not revoke the promise or strike out at her. God keeps the promise through Sarah. Then additional God transformed Sarai's sin into a source of blessing for Hagar and her son Ismael (the name that means God has listened). In the end it is a win-win. Both women become the mothers of great nations.
And before you bring up the fight between the child remember how at the end of the story of Abraham, the sons came together
Then he breathed his last, dying at a ripe old age, grown old after a full life; and he was gathered to his people. His sons Isaac and Ishmael buried him in the cave of Machpelah, in the field of Ephron, son of Zohar the Hittite, which faces Mamre, the field that Abraham had bought from the Hittites; there he was buried next to his wife Sarah.