Foster's Links
Frank's sibling but not from the same father, they are quite suspect at first of the idea of having a sister until there was too much proof that they are indeed related and now feel a moral obligation to watch over her from a distance, keeping an eye on Cardner.
Apparently Frank is their distant sister from another marriage on their mother's side, but barely known of her until after her death. Sadly, they know nothing about how or when she actually passed away.