The Gorgon Sisters' Sisters

The Graeae were two, or some say three, ancient sea-daimones (spirits) who personified the white foam of the sea. They were grey from birth, and shared among themselves a single detatchable eye and tooth.

Perseus & Graea, Athenian red-figure krater
C5th B.C., Archaeological Museum of Delos

The graeae guarded the passage that led to where their sisters were. When Perseus was searching for Medusa, he stole their one eye and one tooth which they shared, forcing them to help him find and kill Medusa. 

Their names suggest rather dire monsters--Deino "the terrible." Enyo "the warlike" and Persis "the destoyer."

The Graiai were usually depicted as old crones. However according to Aeschylus they were Seiren-shaped monsters with the head and arms of old women and the bodies of swans.

Perseus Returning the Eye of the Graiai, Henry Fuseli.