Hannah is pretty and pink faced and a little bit silly. She wore her hair in pigtails for years. At sixteen she still trades chocolate frog cards in front of the common room fire. Ernie never uses sarcasm on her because Hannah will believe anything, for a moment.
That's why it's funny that she doesn't believe them. Of course her mother's not dead! She's gullible, but not that gullible. Then she realizes it's true.