I would take both my hypothetical sib and their partner to get them tested. I'd get tested, too, just to show them there's nothing wrong with being safe. My secret hope, of course, would be to get them to reconsider based on all the information I'd push their way, but if that's against the rules of the scenario I'd also do whatever I could to make sure that they were at my place or I was at my sib's (a.k.a. my parents') place when it happened. I know that sounds kind of weird and creepy, and I don't mean I'd want to be close enough to overhear things, but I'd want to be there in case of anything and I'd want my sib to know that I'd be there for support. Or, if that wouldn't fly, I'd want to make sure there was someone I trusted around.
What I'd really want to do is to kill the mood with education and make sex seem unsexy. If I had any authority or leeway I'd try to make my sib start doing their own laundry and all other kinds of chores with the whole you're-an-adult-now schtick. The chores thing I'd want to do not as some kind of punishment, but because I'd hope it would resonate in a more meaningful way than just telling them that there are consequences. Even people in their 20s regularly ignore possible life-changing consequences when it comes to sex. I just would want to get through to them that it's not all fun and games.