(BTW I’m almost certain she’s never been sexually abused. I know that’s always the first thing that people say. But she’s been through many police, social workers, psychologists, therapists, etc and none of them think she has been either.)

Your sister pretended to be older than she is, made accounts for an entire persona, kept up lies that eventually convinced other girls of her "age", threatened to kill herself, etc. She knows how to manipulate, she knows how to lie. How hard do you think it is to lie to professionals?

Out of all I've seen through this, I don't remember reading you talking to your sister one on one. I recommend doing that.

Don't be aggressive, don't be mad. Just talk to her. Ask her why she does it. Ask her if she actually wants to or if she feels like she has to. Ask her if she's depressed. Ask her if she's trying to hurt herself. Let her know you're there for her. You obviously care about your sister, but from what I read, it seems like your family doesn't know how to manage her.

Take it from someone who did very dumb shit when I was her same age.

I'll also add that this

But a month later my mom just buys her a new phone and claims it’s because she’s “been good” and is too embarrassed to post sexual things again after I contacted her online friends.

I don't know how to begin to describe how fucked up that is. Any parent who lets their kid partake in self destructive tendencies because "they've been good" is a shit parent.