A good example of Jig's sarcasm occurs when the American tells her, "You don't have to be afraid. I've known lots of people that have done it."
"So have I," said the girl. "And afterward they were all so happy."
Later he says, "And I know it's perfectly simple."
"Yes, you know it's perfectly simple."
Earlier she says, "They [the hills] look like white elephants."
"i've never seen one," the man drank his beer..
"No, you wouldn't have."
What she means is not that he wouldn't have seen a white elephant but that he wouldn't have seen a hillthat looked like a white elephant because he doesn't have enough imagination to see how a hill could look like a white elephant.
In all three of the above examples, we can detect a note of sarcasm in her voice. Hemingway rarely if ever describes how a character says anything. He only uses "he said" and "she said," and often he doesn't even indicate who is speaking because it isn't necessary. But his dialogue is so good that we can hear the tone in which the character is speaking, as in "No, you wouldn't have." Sarcasm is always a matter of tone, as in "And afterward they were all so happy." The word emphasized in that sentence would be "so."
Jig is angry, hurt, bitter, disillusioned, and her feelings come out as sarcasm.