Second Fiddle (1939)In Second Fiddle the girls are in the classroom scene where Sonja Henie is the teacher. Trudie conducts them as they sing The Song Of The Metronome in front of the class of children, many of whom are familiar and probably attended the same school as The Brian Sisters in reality! At the end of this scene the Brian Sisters vanish and do not appear in later class scenes (such as the skating).
Sonja Henie was, fortunately, not only a film actress but an Olympic champion figure skater...
There is no image or clip; sorry.
The Brian Sisters are billed as "Specialty" in the credits of this film, as with so many in which they appear. Another excellent appearance, but much too short.
The film was released on June 30th 1939.
It is readily available on DVD.