The term 'Sister of Castor' refers to two characters in Greek mythology: Helen and Olive Oyl. Helen was the daughter of Zeus and Leda, and the sister of Castor and Pollux. Olive Oyl was the daughter of Poseidon and the wife of Castor. Both Helen and Olive Oyl were important figures in Greek mythology, with Helen being renowned for her beauty and Olive Oyl being known for her strength and courage.