Kentucky Moonshine (1938)
In Kentucky Moonshine the Brian Sisters join The Ritz Brothers and Tony Martin.
This film was released on 20th May 1938. In it the girls sing in Moonshine Over Kentucky and Sing A Song Of Harvest. They sing backing phrases to the stars of the film, rather than singing on their own as in most films.
In Sing a song of Harvest the Brian Sisters stand next to Marjorie Weaver but hardly sing at all – even then they only have uninspiring parts.

This photgraph is of Doris, Gwen and Betty accompanying Tony Martin in Moonshine Over Kentucky.
Although the girls sing in their normal, highly competent way and clearly enjoy accompanying handsome and talented Mr Martin, it seems to me as if the few actual words that they are given are intended to be remembered through to the Ritz Brothers parody at the end, rather than being part of a good vocal arrangement.
As a showcase for the Ritz Brothers it may be good, but, try as I may, I really don't enjoy this performance by the Brian Sisters. Sorry, Doris, I know you like it!

At the very end of the film, the Ritz Brothers appear in dresses and Mary Jane shoes, parodying the appearance and singing of Betty, Doris & Gwen.
The accompaniment vocal parts that the Brian Sisters have in this film are competent but less interesting than their work in most of the other films. Their performance with Tony Martin in Music in my Heart (1940) is much better.
That they have been 'set up' for parody by the Ritz Brothers doesn't improve things.
You will enjoy this film overall, but may not be as impressed with the Brian Sisters' appearances as you would expect. If you do see it, please tell us your thoughts!
The film may be available on DVD with some difficulty.