Stefanie Brüll was born into a wealthy Jewish family as the elder of two daughters. Her grandfather, Adolf Brüll, was the owner of Adolf Brüll & Söhne, a shipping, commission and collection agency in Bielitz, Silesia, and her father, Alfred Brüll, was the director of a transport company in Vienna's first district. Her mother, Eleonore, née Steiner, came from a middle-class Viennese family. In August 1909, Stefanie converted to Protestantism together with her future husband, Stephen (Stefan) Demeter (1879–1936), a trade delegate of the Hungarian government who came from Györ. They married in November of the same year. The couple lived in a villa which had been owned by the Brüll family since 1875 and was located opposite Gustav Klimt's studio in Unter St. Veit in Hietzing, Vienna's 13th district. During his worldwide business trips, Stephen Demeter collected hundreds of ethnological objects as a hobby. These trips and his interest in ethnographic objects brought him and his wife Stefanie into contact with scientists at the Museum of Ethnology (MVK) in the early 1920s. Over the years, the Demeters repeatedly donated valuable objects to the museum, such as a “Papuan head” in 1924, which was renowned for its rarity and was exhibited in the New Guinea collection. From 1920 onwards, Stephen Demeter ran an agency and commission business in Vienna, where his wife Stefanie was registered as an authorised signatory. Due to the tense economic situation of the interwar period, Stephen and Stefanie Demeter had to change their office address several times and eventually give up their villa. The move to an apartment had an impact on the large ethnological collection. In 1934, despite his precarious situation, Stephen Demeter donated another 11 objects from Japan, Ceylon, the Solomon Islands and Indonesia to the MVK, these were inventoried as Post XX and Post XXI/1934. In 1935, the Demeters gave the museum 23 different packages (boxes), an ancient Chinese bronze vase from the Han period, plant pots and seven Peruvian antiquities, this time on loan for five years, as well as 60 ethnographic books. The transfer of 24 wooden boxes containing slides and film negatives, 400 passe-partouts and two folders with lecture manuscripts, also on loan for five years, is undated. Stephen Demeter donated three boxes of photos from Guatemala, Peru and Bolivia to the director of the MVK, Fritz Röck (1879–1953). Shortly before his death, Stephen Demeter sold 16 ethnographic objects and 64 volumes of ethnographic books to Honorary Consul Friedrich Wolff-Knize (1890–1949), who donated them to the museum, including several pieces described as valuable and rare, such as a churinga, a giant whirring stick from Central Australia. Stephen Demeter died in June 1936 at the age of 56. Since no other relatives or a will could be found, it can be assumed that Stefanie was his heir.

After Austria's annexation to the German Reich, Stefanie Demeter had to move again. From 16 July 1938, she was registered as a subtenant at Neubaugasse 56/3/4, in Vienna's 7th district. She donated another 351 ethnographic objects to the museum on loan, including mother-of-pearl works from the Solomon Islands, a Japanese wooden mask, dance masks from Ceylon and New Guinea, lacquer objects and much more. In 1940, the museum inventoried 12 objects from the list as gifts under Post XII/1940. Stefanie Demeter was supported by her younger sister Melanie Adler (1888–1963), who lived in London. A sale of the collections on loan to the museum and some other objects did not materialise, and she was unable to carry out her planned escape to England. From August 1941, Stefanie Demeter was registered at the collective apartment at Müllnergasse 33, in Vienna’s 9th district, from where she and 18 other people were deported on 27 May 27 1942. She was murdered in Maly Trostinec on 1 June 1942.

On 14 December 2005, the Art Restitution Advisory Board decided to return the 12 objects that had come into the MVK’s possession in 1940. Stefanie Demeter's legal successors were also informed about the objects on loan to the museum. Many of the objects, books and photographs handed over to the MVK by the Demeters can no longer be found or identified as such due to insufficiently specific descriptions.