Gleaned from brief references around the Internet (April 2022), Flora (1880-1952) and May Hengler (1884-1965) were American singer/dancers.
They have been featured on 2 works published between 1898 and 1901.
Of interest is their Filoscope Skirt Dance by the Sisters Hengler (1896) made by Robert Paul (1869-1943. A Filoscope is a type of flip book fixed in a tin plate holder, an iron lever being used to flip the pages through the viewing position.
At the turn of the century the they were well known on both sides of the Atlantic and continued to perform into the 1920s.
They are quoted as having a good stage appearance singing national airs which have an appropriate dances for each nation.