Why is Mary Carson jealous of Meggie in the novel, The Thorn Birds?

In her sixties as the novel opens, Mary lusts after Father Ralph de Bricassart, who is only twenty-five. When she is unable to entice Father Ralph into a relationship, she becomes intolerably jealous of his affection for little Meggie Cleary, even when Meggie is a child. Ultimately, Mary forces Father Ralph to choose between his ambitions and his growing love for Meggie.