I was the same age when I first travelled a few hours to meet someone I met online. Same country— but still. I made sure to tell a friend his name, where I was going, train details, etc. My mom still found out anyway (long story) and went ballistic. She made me feel ashamed.

I advise you to speak with your sister, on a level. ‘I am saying the following because I care about you and I want you to be safe. I think you’re going to meet someone you met online etc etc. I’m concerned because of the risks associated (elaborate calmly, if you wish) but if you’re going to do this anyway, can you please share as many details with me as possible and check in with me regularly? What is his address and do you have a photograph? Etc.’

By the by, my partner and I are still together over a decade later, so it turned out okay in that respect!