At Himmler’s suggestion, Walter Schellenberg meets with Frl Erna Hanfstaengl. Erna owned a shop in Munich and was the notorious Putzi Hanfstaengl’s older sister, former member of Hitler’s intimate circle of friends, who had flown away to Zurich and then London in 1934.
Because of her brother’s exploits Erna Hanfstaengl had received a great deal of attention from Hitler and there had been, at a time, rumours of marriage between the two. She was claiming to be on familiar terms with W. Churchill and acquainted with Unity Mitford. She was furthermore very close to Himmler’s wife who drew her husband’s attention on her friend’s ability to undertake peace talks in proper circle.
At the meeting she exposes her plan to WS: Hitler would be arrested by the SS and kept in custody with full status but no power. Himmler would form a new government, which was to include General Direktor Schmitz of I.G. Farben and Staatssekraeter Popitz (After the 1944 bomb attempt against Hitler’s life, Popitz would be juged and sentenced to death with Dr Langbehn).
She was to open an art gallery in Paris and wait for the coup by establishing contacts with Randolph Churchill. She wrote several time to her brother interned in Canada but the FBI could not trace any sign of invisible ink or code of any kind.
Source: Part 1A p.63-65

Following this meeting, she flew twice to Paris in July and September 1943 and established connections with a certain Mme Lussac and a former commandant of the French police (name unknown) who had been taken into custody and recently released.
WS had frequent conversations with Frl Hanfstaengl over 1943. He arranged for her to travel despite the restrictions and advanced her 500,000 FF by way of a loan for establishing her art shop in Paris. WS also saw her four times in Berlin. She failed securing the contacts and as she had a somewhat hysterical temperament, he did just what was necessary to please Himmler.
Source: Nara: Final report on WS p 44-45

Frl Erna Hanfstaengl’s mother was born Catherine Sedgwick, of an old well-known New England family, and her father was a wealthy businessman in the art reproduction in Munich, but her strength laid in her brother Ernst. Born on February 11, 1887, Ernst was to years younger than Erna. After schooldays friendship with Himmler's father at the Royal Bavarian Wilhelms gymnasium, Ernst Hanfstaengl’s student friends from his Harvard University 1905 graduation year were future outstanding figures like T.S. Eliot, Walter Lippman, John Reed, and Theodore Roosevelt (Franklin D. Roosevelt graduated 6 years after him but got to know him at the Harvard Business Club).
After graduating at Harvard University in 1909, Putzi (Hanfstaengl’s nickname from governess time) returned to Munich where from October 1909 to October 1910 he served his time as volunteer in the Royal Bavarian "Lieb-Regiment" under His Royal Highness Crown Prinz Rupprecht of Bavaria. In 1910-11 he took over the New York City branch office of Franz Hanfstaengl, Fine Art Publishing House at the corner of 45th Street and Fifth Avenue in New York, the family art business. It seems to have been a delightful combination of business and pleasure. During this time his brother got killed fighting for the Kaiser during World War I. He says himself that, “the famous names, who visited me were legion, Pierpont Morgan, Toscanini, Henry Ford, Caruso, Santos-Dumont, Charlie Chaplin, Paderewski, and a daughter of President Wilson”. Unfortunately his business was liquidated by order of the Alien Property Custodian in the spring of 1919. On February 11, 1920, his 33d birthday, he married Helen Neemeyer, a German-American lady of Corona, L.I., N.Y. A year later his only son, Egon Ludwig Sedgwick Hanfstaengl, was born in New York City, February 3, 1921. A daughter, Hertha, who died when a few years old, was born to them. In the summer of 1921 Mr. Hanfstaengl took his wife and son to Munich and there, introduced by Captain Truman-Smith, the U.S. Military Attaché in Berlin, Putzi Hanfstaengl became an ardent Hitler supporter, his Foreign Press Chief.
Source: Hitler’s Secret conversations. p.458
Francis Morris Sedgwick & Hubert Merrill Sedgwick: A Sedgwick Genealogy Manuscript to about 1950 New Haven Colony Historical Society, MMS-B46 Box 6 Folders B – R; extension of the 1934 Francis Morris Sedgwick manuscript A record of descendants of Samuel Sedgwick, 1667,
Ernst Hanfstaengl: Unheard Witness. J. B. Lippincott Co, Philadelphia & New York - 1957.
Ernst (”Putzi”) Hanfstaengl: Hitler. The missing years. Eyre & Spottiswoode, London - 1957. Same text as Unheard Witness
Persico, Joseph E. Roosevelt's Secret War: FDR and World War II Espionage. Random House - October 9, 2001 p. 192

Ce n'est pas une preuve mais c'est une source (pas Erna, l'info)