I'm somewhat familiar with Japan's "pinky violence" films from around the same period of time that Big Bad Sis originated, but I really don't know if Hong Kong and the Shaw Brothers studios had a similar run of films of this nature. Big Bad Sis is certainly in the same family of movies.
The film walks an odd line between being trashily exploitative and being a tale about women who've been abused and treated poorly standing up for themselves and fighting back.
The main person responsible for the women sticking up for themselves is Ah Sing. She's a quiet but fiery woman who doesn't sit idly by while bad things happen to others. She says she doesn't know kung fu, but she fights like a madwoman, smashing and slashing her opponents with anything at hand and never giving up. She's also the first to pitch in and organize assistance whenever one of the factory girls need help.
It's not long before two of the other girls at the factory she works at are following her around and asking her for a place to stay to escape their own horrible home lives, and despite her initial hesitation she gives in. She has her own dark past that she's not very interested in talking about, but she's happy to help them begin training in basic exercise and self-defense. Ah Sing is no master of the martial arts, but she makes up for it with a high level of enthusiastic savagery and a practical working knowledge of how to hurt people.
Pretty soon the girls get a chance to try out what they've learned. Ah Fong's step-father and would-be rapist won't leave her alone. Ah Sing tells her to go out and deal with him. And with a little help from her friend, she gives her step-father and his sleazy friends such a vicious beating that he promises to not only stop bothering her, but to stop hitting her mother as well. It's an important moment for Ah Fong, who's been downtrodden her whole life.
Ah Sing quickly becomes known as the go-to girl at the factory for anyone who has a problem. Sister been sold into prostitution? Talk to Ah-Sing! Can't afford to get married and move into an apartment? Talk to Ah-sing! Get seduced and knocked up by a rich lecherous playboy who is taking advantage of all the poor factory girls? Losing the factory and everyone's jobs because you're a gambling addict? Who you gonna call?
Unfortunately a number of these issues put Ah Sing on path that lead to conflict with the dark past she'd worked hard to leave behind, and it's only a matter of time before her continued hard-headed interference has her headed for a showdown with some criminals.
Big Bad Sis has a fair amount of exploitation material. There's quite a bit of nudity, a sex scene, a mostly topless fight scene (which is a pretty good fight nonetheless), women fighting women, and women fighting men. While everything is in context to the story to some extent, it certainly becomes gratuitous in numerous parts. I felt that it was kind of at odds with the central theme of one particularly strong woman standing up and living life her own way, and helping other women do the same. Despite that I enjoyed the movie- and I am sure those aspects will make the movie even more appealing to some folks.
It also has some interesting characters, and Ah Sing in particular is just a cool bad-ass heroine. There are also some good fights; not conventional kung fu fights, but huge brawls with some kung fu. The final showdown is also quite good.