Some interesting background in this.

Moucher, the Duchess Dowager of Devonshire, (née Lady Mary Gascoyne-Cecil) was the daughter of the 4th Marquess of Salisbury and sister to the 5th Marquess, who carried the Sword of State (later the Jewelled Sword) at the Coronation. The Cecils wielded huge political influence that can be traced back to Robert Cecil, Elizabeth I's Minister. They still do today and in the earlier years of the present reign, Robert Gascoyne-Cecil (the 5th Marquess) led the manoeuvres that ended in the appointments of both Macmillan and Douglas-Home as Prime Ministers. In 1953, he was Lord President of the Council. On this thread, the Marquess of Salisbury is seen standing to the right of The Queen, carrying the Sword of State.

Mary Gascoyne-Cecil married Edward Cavendish (10th Duke of Devonshire from 1938) in 1917. The 10th Duke died in circumstances that remain mysterious in 1950, a suspected victim of Dr John Bodkin Adams. Their elder son, William (killed in action in 1944), had married Kathleen, sister to John Fitzgerald Kennedy. Their younger son, Andrew, succeeded as 11th Duke in 1950, having married Deborah Mitford in 1941. Deborah, the daughter of the 2nd Lord Redesdale, is sole survivor or the celebrated Mitford girls, who included among her brothers-in-law the notorious Oswald Mosley.

On his father's death in 2004, Peregrine (Stoker) Cavendish succeeded as 12th Duke of Devonshire. Mary Cavendish continued as Mistress of the Robes until 1967 and was also Chancellor of the University of Exeter. She died in 1988.