NEI. You never even met this guy. Makes it pretty hard to judge what he's like. Makes it a lot harder for Redditors to judge this situation. The STD thing is fucked up, but we don't even know how that happened. Did he know he was infected and not tell her? Was the sex consensual? Why are they not using protection? Questions like this make it hard to judge a person we don't know anything else about, other than that he's significantly older. Which doesn't have to be a negative thing, because please remember how shitty teenage boys were back then. I don't know how old you are and how old the guys are you are dating. In my experience men are often turning into more compassionate creatures once they passed age 30... Nevertheless, we really know nothing here. And you don't want to meet this guy either. So you should probably let your sister do her thing. 18 year olds aren't as stupid as they behave sometimes. People in love are stupid though, that's true.

If you are really worried, you should probably give it a shot, and find out who that person is.