London's daughter-in-law really doesn't play much of a role in this novel—we don't care how much hype Selena Gomez is getting for playing the role in the 2016 movie adaptation. She's basically there to provide an opportunity for Mac to win London over, a commodity to be used for the cause:
"We've got to use whatever material comes to us. That was a lucky break. We simply had to take it. 'Course it was nice to help the girl, but hell, even if it killed her—we got to use anything." (48)
Lisa's little more than a person with a pulse to whom Jim can talk sentimentally when he needs it. Other than that, she literally nurses her baby. The. Entire. Time.
But Lisa also represents something bigger than herself: the hopes and dreams of all the workers in the camp, before life has had a chance to beat those hopes and dreams out of them. Doc Burton gets at this little spark of Lisa's humanity when he asks her what would make her happy now. She tells him that she'd like a cow, just as she had when she was a girl:
"Old man used to milk it into a cup-like, to drink. Tasted warm. That's what I like. Bet it would be good for the baby." (198)
In addition to her cow, Lisa would like a toilet close by. Her desires are pretty basic, but in this world, she might as well be asking for the moon.
Lisa's memory also touches on the loss of past prosperity that was the common lot of many of the migrant workers Steinbeck wanted to depict in his work. Lisa remembers the days when her family did have a cow, when a child did have access to healthful, warm food that would help it grow.
Her passive and listless existence—she's reduced to little more than daydreams and basic bodily functions—represents all the lives barely being lived in the squalor and despair of the camp.