So, all I have is your bias story to go on, (sorry but there’s no objectivity to pull from a one sided story) but if you’re right, it sounds like this “friend” has something resembling a narcicistic personality disorder or something like it. If that’s true, it’s something to worry about. People with a condition like this tend to lure you in with kindness, force you to do stuff for them with the so called “kindness” as leverage, but at the same time destroying you by feeding negativity to you and the people around you about you, to maintain control over you by playing with you insecurities. And with “you”, I mean your sister in this case. Your sister will just be a pawn in their game to get status, attention, control, which will result in your sister getting hurt a lot and them moving on onto another victim.

But there’s a chance this is all wrong, again, I’m basing it only on your story. I don’t know if you are seeing everything which is happening clearly. Cases like this are always very difficult.

Maybe you can advice your sister not to get too dependent and attached on this “friend” and to be careful. And be there for her when it goes south. Don’t judge her, she’s a victim. Be supportive. Other than that, I doubt there’s anything you can do. It’s something which will have to play out as long as your sister isn’t ready to take her distance. It will be a hard life lesson, but a valuable one. Prep her with your insight, let her make the choice.

That’s my advice.