ca. 1565-1568 Margaret Clifford, Countess of Derby, or a Lady of the Wentworth Family, by Hans Eworth
According to "Prince_Lieven" on the Alexander Palace Time Machine Discussion Forum, Lady Margaret was apparently a flightly, unreliable sort of character, not really taken seriously as a claimant. She was outwardly a Protestant, but apparently had Catholic tendencies, like her husband. Their son, Ferdinando Stanley, was approached by Catholic conspirators about claiming the throne, but he sensibly reported it to the council.