Honestly @Pan and @Zaboem hit it on the ball.
Right now, I am seeing this as a situation where you might have to prepare for the worst case (just IN case). If you still can, still readily try to convince them otherwise. Honestly? @Zaboem might be on to something. Use different word choice when speaking with them -- give them a different reason to not go through with this decision.
But as you're doing this -- have constant contact with your sister. Have back up contacts for her so she can get to you. That can be social media, phone, text, email, whatever.
Also prepared to keep some of that contact a bit more private. From what you're telling us now, at least some of your family and those around you who think this way are assuming you're leading your sister down some "dangerous path". But you know your sister. You know she's scared right now -- and she was scared enough to seek your help.
And sometimes, helping is doing so where those offenders are not aware of it.
And I know you're worried now. I can see it in your words. Right now -- you need to take a few breathers. Just so that you can remain as calm as you can while responding and so that you can think this through.