The story of the sons of Abraham is a fascinating story. I sympathize with Hagar and her son Ishmael, because Hagar (being a slave) did not really have a voice in Sarai's plan. She was compelled by her master to give birth to Ishmael. But then, when the angels descended to Sarai and told her she would conceive despite her advanced age, she gave birth to Isaac. Then, with a son of her own to carry on the name, she no longer needed Ishmael (or Hagar) so she basically just threw them out like last week's garbage. It's a sad tale.
Sara is the problem. She jumped the gun and overrulled Gods plan. God told her... She would be the mother of the covenant one. Poor Hagar did what she was told. GOD had a plan and Sara blew it. BUT the promise was for Issac ... Just like what GOD said. Ishmael got the oil. Issac got Israel.