Who is Miss Duncan the First and Miss Duncan the Second from Elijah of Buxton and what is their importance?

When the Taylors arrive at Buxton, the Duncan sisters realize that the new arrival, Liza Taylor, is actually Alice Duncan, their blood sister, who was sold away from the family when she was young. They identify her by a scar she got as a child. They decide to wait a while to tell Liza this news, and Elijah is confused about why grownups would wait to tell such a thing.