why does Momma and Bailey not notice Marguerite's sudden decision to start talking again?

They don't notice she's begun talking because she only talks for Miss. Bertha, and in the beginning only speaks by reading books aloud. At first, she has no words of her own so she only uses the words of others......... and she only uses the for Miss. Bertha Flowers.

Note; the first time she does speak with her family, she gets herself into trouble with her mother!