Boyfriend's (32M) younger sister (16F) involved in weird cult-like clique, he asked me (27F) to help but I have no idea how to deal with this situation

So this is kind of a unique situation. My boyfriend has a very young sister. Their mother passed when she was 2, and their father is a nice guy but a total workaholic and is rarely home. My boyfriend lived about an hour away and tries to be a good brother but obviously he can't be everything for her.
Recently he's found out that that she is part of a 6-member clique at her all-girls high school, and none of it seems healthy. We found her real instagram and they all have like "aesthetic" type accounts, which is not weird by itself, but they all consider each other sisters, have some kind of seniority system, and also harshly punish each other if they do something "wrong." He told me that they will physically fight each other outside the grounds of school for "offenses," they monitor each other's weights and appearance (they have like weigh-in days or something, have to wear certain things, look presentable and neat, etc).
My boyfriend feeling totally overwhelmed by all this asked me to talk to her female-to-female. I'd been through my fair share of clique-y behavior in high school so I said I'd gladly try to help or mentor her if possible. I'd met her before and she was a nice girl.
When I talked to her, she was really open about her friendship with these girls and it was just so... strange. It honestly feels like some kind of cult. It's beyond my scope of cat-fighting and gossiping. They aren't really like negative in the grand scheme of things. They push each other to do well in school, treat each other with respect, be healthy, and look presentable and neat. That's good right? But if they don't follow some of these "rules" they'll physically attack each other by slapping , hitting, ganging up on one girl. And she just seems like totally jaded to that, like it's just normal behavior and she doesn't think any of it is wrong at all.
I'm kind of at a loss what to say or do. My boyfriend is really worried but I think he's thinking maybe I'll be a good influence on her. I don't think this is a situation where being a good or bad influence really matters. She doesn't think anything is wrong, but the whole situation is really bizarre.
What exactly should be done here?
Tldr: my boyfriend's younger sister is part of a weird cult/clique with 5 other girls where they hold each other to really high standards, considered themselves sisters, and also physically hit each other for infractions. I don't know what the hell to do or how to help. How can I help?


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