8 The child grew and was weaned, and on the day Isaac was weaned Abraham held a great feast. 9 But Sarah saw that the son whom Hagar the Egyptian had borne to Abraham was mocking, 10 and she said to Abraham, “Get rid of that slave woman and her son, for that woman’s son will never share in the inheritance with my son Isaac.”
11 The matter distressed Abraham greatly because it concerned his son. 12 But God said to him, “Do not be so distressed about the boy and your slave woman. Listen to whatever Sarah tells you, because it is through Isaac that your offspring will be reckoned. 13 I will make the son of the slave into a nation also, because he is your offspring.” Genesis 21:11-13
Abraham had a couple of problems on the homefront. Both sons were living with him but Sarah was angry at Hagar and Ishmael. Now that Sarah had a son, her hatred towards Hagar grew stronger. Even though she had told Abraham to sleep with her, she must have been regretting that decision by this time.
This is merely a reminder that while sin can be forgiven, its consequences can linger for years. Sarah took out her anger on Hagar, but ultimately she should have blamed herself.