Tricky one this.
I can't completely speak against it because I met my boyfriend on the internet. After a month and a half of talking online we figured out we had chemistry and got it together. He told me he loved me after a week and that he wanted us to live together after two weeks. Sounds mad, yes, but the feeling was entirely mutual. We met for the first time when I flew to meet him at CDG airport in Paris, where he was studying at the time. That was in Feb 2005 and we've been together ever since, so I can't knock meeting people off the internet completely.
However, as I did, your sister should take necessary precautions. I'd be worried about her too. It's good that she's asked you to come with her but maybe once she asks you to leave you could hide somewhere where you could still see them? I know it seems underhand but at least you'd know she's OK.
Sorry I don't know how else to help. Hope it all works out.