Miriam and Mary have similar experiences during the novels, such as rejection and jealousy. They both feel a similar pain when the objects of their affections choose younger, beautiful women. Though they experience the same kind of pain, their reactions to it are markedly different.
Miriam from A Thousand Splendid Suns has a promising marriage until she has the first of several miscarriages. Her husband takes a second wife, making Miriam unhappy and jealous. However, she warms to her rival, Laila, when the younger woman stands up for her. Miriam becomes close to Laila and treats her and her children like family.
Miriam’s initial approach to her betrayal mirrors her mother’s advice:
A man's heart is a wretched, wretched thing. It isn't like a mother's womb. It won't bleed. It won't stretch to make room for you.
Mary Carson is the wealthy aunt of the protagonist, Meggie Clearly, in the novel The Thornbirds. Meggie moves from New Zealand to Australia because Mary offers her father a job. Mary is very interested in Ralph de Bricassart, a disgraced priest living in the area, and attempts to seduce him. He, in turn, is only interested in her for her money, hoping she can buy his way back into favor in the Catholic Church. He does not respond to Mary’s overtures. Instead, he focuses on Meggie, which makes Mary jealous.
Miriam and Meggie are similar because they are both rejected by men they care for. They also have to cope with the objects of their affections choosing younger and more attractive women. They differ in how they handle jealousy. Miriam is angry, but after her change of heart, she loves Laila and goes to great lengths to defend her instead of trying to destroy her husband’s relationship with Laila. She becomes kind, gracious, and self-sacrificing. She meets a tragic end in a peaceful state of mind:
And yet she was leaving the world as a woman who had loved and been loved back. She was leaving it as a friend, a companion, a guardian. A mother. A person of consequence at last.
Meggie reacts very differently to her rival, her niece Meggie. Her jealousy makes her angry and vindictive, and she takes active measures to keep Meggie apart from Ralph, the object of both of their affections. She tells Ralph:
I'll pin you to the wall on your own weakness, I'll make you sell yourself like any painted whore.
She forces Ralph to choose between Meggie and the money. She succeeds in separating the two because Ralph is weak enough to choose the money and his career instead of Meggie.
Mary uses her fortune to tempt Ralph away from Meggie and to ultimately separate the two lovers. She uses Ralph’s weaknesses to doom the couple. She is neither loving nor self-sacrificing.