Who is Mrs. Murdo from The People of Sparks and what is their importance?

Mrs. Murdo is a a mother to Lina and her sister, Poppy. She stays with the girls at Dr. Hester's, and she takes special interest in the medicines and treatments the doctor provides the villagers. When the decision is made that the people of Ember will remain in Sparks, Mrs. Murdo becomes Dr. Hester's assistant.