There's a set iconography. Costumes or accessories that immediately evoke imagery and associations. For countless idol groups, the go-to iconography is the Japanese schoolgirl and all the youthful associations she entails.
Not the Sexy All Sisters. They largely eschew schoolgirls in favor of superheroes, juvenile delinquents, and waitresses.
The group was born out of low budget Japanese cinema. It all started with the announcement of exploitation flick Bakunyuu Sentai Pai Ranger (Huge Boob Sentai Tit Ranger). Yuu Tejima, its star showed up in superhero gear with her squad of well-endowed super heroines. The group sang the flick's theme song.
Bakunyuu Sentai Pai Ranger wasn't pornography, but a tongue-in-cheek Benny Hill style parody of Super Sentai (Power Ranger) flicks. The song's video featured the Pai Rangers in low-cut outfits, singing boob pun after boob pun. It's lowbrow stuff, but that's the point.
The umbrella group Sexy All Sisters expanded, creating more silly spin-off pop groups, such as the Dynasty Warriors-like "Bakunyuu Sangokushi" or "Annals of the Three Busty Kingdoms" as well as Bakunyuu Yankii—a juvenile delinquent inspired group—and Bakunyuu Koshien—a baseball inspired group. Koshien is a legendary baseball field in Japan. There's also a group that wears Anna Miller's waitress outfits. It's like they're going through a cosplay checklist, ticking off every box.
The girls aren't very good singers at all—but many "real" idols aren't either. What is refreshing about them is that there's a complete and utter lack of pretense with the Sexy All Sisters. Everything is very blunt, instead of playing coy with sexuality, like so many Japanese pop groups tend to do.
Anime and games regularly feature female characters with large breasts, but they often end up being window dressing—not the actual window. This actually makes the characters far more passive; they're unable to call people out for leering at them, because they seem completely unaware of their own appearances. However, Sexy All Sisters release songs—not very good ones, mind you—in which they call out the perverted listeners for looking at their chests, instead of just standing there passively for people to gawk and gaze. It's overdone and exploitive, but oddly empowering.
Check out more at the group's official site.