The wife of the owner of the tavern where Robert Finch takes Isabel and Ruth to be sold, Jenny is a welcome source of comfort. A close friend of their parents from long ago, Jenny cared for the girls when they were younger and is the reason for Isabel's practically photographic memory. She tells her:
"We used to make a game of it. Tell you a line to memorize, or a song. Didn't matter how much time had passed, you'd have the whole thing in your mouth." (3.43)
Think about how many times Isabel has to memorize things in this book—from the code to get into the rebel camp to the words of Lockton's Loyalist conversations. In a small way, Isabel has Jenny to thank for that.
An indentured servant from Ireland, Jenny is very familiar with the social hierarchy of the time. This is why she refuses to buy the girls when Isabel asks if they can stay with her—doing so would mean standing up to upper-class folks she's forbidden to associate with. It's only after she hears Madam Lockton begin to make demands on the girls that she steps up and says she'll buy them. It might not seem like much, but this is a gigantic risk. As Isabel says:
A person like Jenny did not speak to folks like the Locktons or Mr. Robert, not in that manner. (3.93)
Madam makes a mockery of Jenny's attempt to save the girls by raising her offer and, in doing so, swiftly shutting Jenny out of the negotiations.
So, why do we even have this scene in the book if Jenny totally disappears at the end of the chapter? Jenny's behavior is a clear indication of how deeply social class rules run in this society. Family connections literally aren't worth anything when it comes to slavery—families are split up and people who knew the parents of abandoned slave children don't have the right to take them in. The encounter sets the stage for the parade of scenes that reveal the low position Isabel occupies.