what is ms shelby doing

I'm not sure what you are asking here (she does a few things) but hopefully this quote from Chapter 10 helps.

Mrs. Shelby entered. Aunt Chloe set a chair for her in a manner decidedly gruff and crusty. She did not seem to notice either the action or the manner. She looked pale and anxious.

"Tom," she said, "I come to—" and stopping suddenly, and regarding the silent group, she sat down in the chair, and, covering her face with her handkerchief, began to sob.