I've done it and I didn't get murdered or trafficked,
however I know in hindsight it was an absolutely insane thing to do and very dangerous. I lied about where I was going and didn't tell anyone who I'd be with. I didn't even have a smartphone at the time.
If something bad had of happened to me I would have dug that grave for myself.
Is your sister on the spectrum? I'm super into true crime and I
still didn't fully understand the risk I was taking, but I blame it on my autism because even though I don't think I'm naïve about risk, my mum says I am and this would certainly be an example of it.
The sealed envelope thing is an excellent idea. Get her to do that if you can, or get a responsible adult to have that conversation with her if you think they'll be more persuasive. It's not about an invasion of her privacy, it's about keeping her safe.
She is undiagnosed neurodivergent, she is trying to get an ADHD diagnosis. Wouldn't be surprised if she's also on the spectrum. She wouldn't be surprised either.