You won't be able to do this until you've found out all you can about Mavis by talking to her, Harriet Sherman, Joseph Mitchell, and Mavis's son Sam.
How can I find out more about the ghost?
How do I find Mavis's son?
How do I find Joseph Mitchell?
When you know what gift Sam gave his mother, come back and talk to Mavis. You can now use that information to hook her gently out of her cocoon.
After you knock, choose the options "Sam's present," "the book," "question her mothering ability," and "out in the hallway." Poor agoraphobic lady.
And there's the Countess again!
Who was that old woman?