Who is Ruth from Chains and what is their importance?

Ruth is Isabel's little sister. She is five-years-old and suffers from epilepsy. When the girls' owner, Madam Lockton, witnesses one of Ruth's fits, she is sure the she is devil possessed and immediately tries to sell her. Lockton lies to Ruth about her sister's fate, insisting the girl has been sold, though she was really sent to Charleston awaiting a buyer.