I'm watching Irving Berlin's White Christmas. Near the beginning, the team of Wallace and Davis are performing with their troupe. They are planning to take an overnight train to New York, and Davis wants them to spend the intervening time on a double date. But Wallace says they need to go review an act as a favor to an old army buddy. Davis asks what kind of act, and when Wallace informs him it is a "sister act," he is very disparaging.
My searching for "sister act" turns up nothing except references to the Whooping Goldberg movie franchise. I guess I know why an act was a sister act, but what was so bad about one?
"Sister act" comes from vaudeville and means an act where two or more women are working together, and often were (or were billed as) sisters, often twins or triplets (even if that wasn't the case); usually they would sing or dance. Sister acts sometimes featured risqué outfits and bordered on burlesque, with the main attraction usually being the attractiveness of the "sisters" more than their skill - hence, not a good place to take a date.
This wasn't always the case, however, and several very talented influential musical groups began as sister acts, including the Boswell Sisters and the Andrews Sisters. Descendants of the sister act concept can be seen today in musical "girl groups" and unrelated women being cast and partnered together as "twins" or "sisters" in adult entertainment. You can read a bit more about it in Sister Sets: Sisters Whose Togetherness Sets Them Apart.
Thank you. Very informative.