In Lyddie, what dreadful news does Lyddie receive in chapter 20?

Lyddie gets a letter telling her that her mother died.

Since she was a little girl, Lyddie wanted nothing more than to get her family back together.  Her father left when her youngest sister Agnes was born, and her mother was never the same after that.  She slumped deeper into depression until she left the farm to go to Lyddie’s uncle’s house, taking Lyddie’s two little sisters with her and leaving Lyddie alone with her...

Lyddie gets a letter telling her that her mother died.

Since she was a little girl, Lyddie wanted nothing more than to get her family back together.  Her father left when her youngest sister Agnes was born, and her mother was never the same after that.  She slumped deeper into depression until she left the farm to go to Lyddie’s uncle’s house, taking Lyddie’s two little sisters with her and leaving Lyddie alone with her brother.

After taking the job at the factory, life was full of loss for Lyddie.  First she learned that her youngest sister Agnes died.  Then her uncle came to tell her that her mother was being sent to an insane asylum.  That left Rachel, who stayed with Lyddie until her brother Charlie came to take her home to his new family.

After Rachel left, Lyddie began to realize that she had to let go of the dream of getting her entire family back together again.  Agnes was dead.  Charlie and Rachel were adopted, her mother was institutionalized, and no one had heard from her father for years.

Then Lyddie got a letter saying that her mother had died.

A letter arrived in September, on thick, expensive paper, the addressdecked out in curlicues. "We regret to inform you of the death of Maggie M. Worthen ..." They hadn't even got her name right. Poor Mama. Nothing ever right for her in life or death. (Ch. 20)

Lyddie’s mother’s name is Mattie, not Maggie.  Lyddie thinks to herself that her mother has been gone from them for years.  This is just the final loss.

The only family Lyddie really has left is Brigid.  Although they are not related, they are close.  Diana is gone, and Lyddie enjoys teaching Brigid how to read.  One day she goes to find her friend and sees Mr. Marsden, the overseer, trying to assult her.  Lyddie beans him with a bucket and rescues Brigid.  Unfortunately, he trumps up charges against her and gets her fired.