Question: This mansion in St James's, London is the property of a nobleman who was the younger brother of who?

Context paragraphs:
Charles Spencer, 9th Earl Spencer: Charles Edward Maurice Spencer, 9th Earl Spencer, (born 20 May 1964), styled Viscount Althorp between 1975 and 1992, is a British nobleman, peer, author, print journalist, and broadcaster, and was the younger brother of Diana, Princess of Wales.  Through his sister, Spencer is the maternal uncle of Prince William, Duke of Cambridge and Prince Harry.

Spencer House, London: Spencer House at 27 St. James's Place is a mansion in St James's, London, and is the property of the Earl Spencer.

Piccadilly Market: Piccadilly Market at St James's Church Piccadilly is a market in the St James's district of the London borough of Westminster.  It is open six days a week and occupies the courtyard on the North side of St James's Church, Piccadilly.  It is close to Piccadilly Circus, Fortnum & Mason and The Ritz London Hotel.  There are some forty-five stalls.

Mark Masons' Hall, London: Mark Masons' Hall in London is the headquarters of The Grand Lodge of Mark Master Masons of England and Wales, which also controls the Royal Ark Mariner degree.  It is located in 86 St James's Street in the central London district of St James's, opposite St James's Palace.  While Freemasons' Hall is the headquarters of the United Grand Lodge of England and the Supreme Grand Chapter of Royal Arch Masons of England, Mark Masons' Hall is the home of several other important appendant orders of Freemasonry in England and Wales.

Carlton House: Carlton House was a mansion in London, best known as the town residence of the Prince Regent for several decades from 1783.  It faced the south side of Pall Mall, and its gardens abutted St. James's Park in the St James's district of London.  The location of the house, now replaced by Carlton House Terrace, was a main reason for the creation of John Nash's ceremonial route from St James's to Regent's Park via Regent Street, Portland Place and Park Square: Lower Regent Street and Waterloo Place were originally laid out to form the approach to its front entrance ("illustration, right").

St James' Church, Forest Gate: St James' Church, Forest Gate was a church in Forest Gate, east London.  Its origins lay in an iron building constructed around 1870 to serve a conventional district.  A parish was formed for it in 1881 from those of Emmanuel Church, All Saints and St John's and its permanent church completed the following year, with an organ moved from St Matthew's Church, Friday Street.  The church was demolished in 1964 and for two years its congregation worshiped in the Durning Hall Community Centre's chapel until the parish was merged with that of St John's.  A new church was built for the St James' congregation at northern end of St James Road in 1968.  The congregation finally moved to St Paul's Church, Stratford in 2014, though the area that had formerly been St James' parish was instead transferred from St John's to St Saviour's the same year.

St Saviour's Church, Forest Gate: St Saviour's Church is a Church of England church in Forest Gate, east London.  It was opened by Emmanuel Church, Forest Gate as a mission hall in 1880, with a separate parish (split off from that of Emmanuel) and a permanent church following three years later.  Its first vicar was Henderson Burnside, one of the first Anglican missionaries in Japan.  It opened a mission at "365 Railway Arches" in 1903.  Its parish is now known as 'St Saviour's and St James's, Forest Gate' after the area formerly forming the parish of St James' Church, Forest Gate was transferred to it in 2014 - the congregation from St James', however, transferred to St Paul's Church, Stratford, also in 2014.

King Street, St James's: King Street is a street in the St James's district of central London.  It runs south-west to north-east from St James's Street to St James's Square.

Westminster St James: Westminster St James (or St James Picadilly) was a civil parish in the metropolitan area of London, England.  The creation of the parish followed the building of the Church of St James, Piccadilly in 1684.  After several failed attempts, the parish was formed in 1685 from part of the ancient parish of St Martin in the Fields in the Liberty of Westminster and county of Middlesex.  It included part of the West End of London, taking in sections of Soho, Mayfair and St James's.  Civil parish administration was in the hands of a select vestry until the parish adopted the Vestries Act 1831.  The vestry was reformed again in 1855 by the Metropolis Management Act.  In 1889 the parish became part of the County of London and the vestry was abolished in 1900, replaced by Westminster City Council.  The parish continued to have nominal existence until 1922.

St James Workhouse: The St James Workhouse opened in 1725 on Poland Street in the Soho area of London, England, in what was then the parish of Westminster St James, and continued well into the nineteenth century.  The infirmary at St James Workhouse was contemporaneous with the advent of the song "The Unfortunate Rake", the basis of the song "St. James Infirmary Blues", although it is uncertain as to whether it refers to the workhouse infirmary.  Inhabitants included the painter Charlotte Mercier, who died there in 1762.

Supporting paragraphs: Spencer House, London Charles Spencer, 9th Earl Spencer
Answer: Diana, Princess of Wales