Just tell them to use protection... But seriously, if you wont let your kids have some degree of privacy, they will just stop telling you about things and do them anyways without your knowledge. (I know you arent the father, but still) Obviously the parents of both kids need to have talked about it if she goes further away, but as long as you have their contact info whats the problem? At that age i would have laughed my parents in the face if they told me i wasnt allowed to meet with other kids my age where and whenever i please.

They are absolutely gonna want to do sexual stuff, that's the nature of teenagers. Just don't take on anything that you cant supervise to your level of comfort, OP.

Dad's being an asshole. Accompany them on their first IRL date and make sure Dominic is who he says he is and that he's not an asshole.

If her parents don’t want her to have a boyfriend that might be bad parenting, but not something you should get in the middle of.

Is there any reason for them to be worried about Dominic besides age and fear of the internet?

The harder one compresses a spring, the more wild and unpredictable its travel will be when it gets released. Stressing a spring too much results in permanent damage too. If you want to release the tension and avoid damage, one must gradually release tension in a controlled manner.

Your ultimate goal is to allow the little spring to actuate well on its own. She will never do this if someone else is controlling and making rules. Depending on her emotional versus intellectual intelligence and reasoning skills, the best course of action, in my opinion is to explain the situation and constraints. Then clearly explain your expectations. Explain the consequences in terms of the potential impact on her life, and the consequences of disappointing you. Finally let her formulate her own constraints, how she can manage herself, and what should happen if she fails.

Her reasoning about the situation and solutions shows exactly what she is thinking, her real depth of reasoning, and what to watch.

Even at 24, you likely still have a ways to go in self awareness. Humans are not cognitively fully developed until 25. I am forty, and gained much of my self awareness about psychology and functional thought as it differs and relates to other types of people throughout my 30's. In the USA, we focus far too much of the age of majority as some kind of benchmark of adulthood, but never forget, the age of majority at 18 is arbitrary unscientific nonsense and only exists because this is the earliest age when boys are physically developed enough to carry the full kit for professional murderer orgies. It is the age when these kids are most easily manipulated in to stupid situations in support of thugs and corporate criminal enterprise. There are only two relevant scientific epics in our life cycle, puberty and a fully formed brain at 25. The age disparity between these two points is largely why we exist as a species.

We live in a world where having a child is like slavery and a nearly insurmountable expense for most. That is how I would put it. Having a kid will ruin you for life and lead to slavery in all but name unless you do absolutely everything right first and only have a kid when it is planned.

This is a slippery slope situation, but I hate slippery slope arguments.

It's time for a full and frank conversation with your sister.

Everyone who loves her and cares for her is trying to protect her, but ultimately she's rapidly approaching the age where she has some responsibility and autonomy and if she sets her mind to it she will find a way to get in to trouble.

The elephant in the room that's perhaps obvious to everyone but her is that meeting one time and standing around isn't going to satisfy both lovers for 2 years. What is the plan? Where to from here? Inevitably they will want more time together and that's when things will get really complicated. What if she wants to go visit for a few days?

She also needs to know about the imbalance of responsibility between her and Dominic just because sex for him is a brief encounter but for her it carries the potential for a lifelong commitment in the form of a child.

It's hard but really she needs to realise that she's playing a pretty serious game, and her best option is to keep it strictly online for a few more years.

Maybe talk to your dad instead. It's usually better to allow such things and stay in connection with your daughter, rather than force her away and do silly things. And it's not uncommon that people have friends, even boy/girlfriends at some point. I think it's a bit archaic to separate kids based on gender and don't allow any contact. Since like 50 years we've replaced that with education, making teenagers know about consequences and aware of how they're responsible for their own lives.