It's a Great Life (1929)

"Get ready for the greatest treat of your picture-going days! The great Duncan sisters in their first all-talking-singing-dancing picture!"

Casey and Babe are sisters who work in a department store and each year the store puts on a show. As expected, things are going wrong with every act until Casey comes out to help Babe with her song. They are a hit, but in the final act, Casey again comes out and this time the president sees her act and fires both her and Babe on the spot. Benny is able to book Casey, Babe and Dean into Vaudeville and their act is popular. But before they have their shot at stardom, Dean and Babe leave Casey and the act.