|Portrait of a Lady, possibly Margaret|
Margaret Stanley (née Clifford) was daughter of Henry Clifford, 2nd Earl of Cumberland and Eleanor Brandon (daughter of Mary Tudor, Queen of France). This put her next in line to the throne, after the Grey sisters. An attempt was made by Dudley to marry Margaret to his son, Guildford. However, Clifford refused to support Jane Grey as Queen, thus the marriage did not happen. Fittingly, Queen Mary gave Dudley's confiscated jewels and other riches to Margaret upon her wedding to Henry Stanley, heir to the Earl of Derby in 1555.
Because of the Lady Jane Grey scandal, Margaret concluded that the Grey sisters were unfit to rule, as was Elizabeth Tudor (since she was not Catholic). Thus, she was Mary's heir presumptive. However, the Stanley's were not held in high esteem by the people, thus had little backing.
Even when her husband became the Earl of Derby, the couple had financial troubles. Stanley consoled himself with his mistress, while Margaret dabbled in alchemy. This led to her consulting "wizards" which caused her to lose favor with Queen Elizabeth. She really got into trouble when she was accused of consulting a Dr. Randall to see how long Elizabeth would live, causing her to be banished from court, permanently.
Tudor England: An Encyclopedia