As a youth worker I work with 14 year olds (mainly 7-15), and I know that some of them are sexually active, they sometimes feel they need to talk, and because I am 17 and they know me, they approach me with problems from time to time. I have to act in their best interests and my duty of care to them would extend to writing a report to the Group Leader and next year when I'm 18- making the police aware of the facts, thankfully that Is something I have not needed to do, and I hope I never will, but if they did say anything, then they would leave me no choice.
Remember she is, by law, incompetent to give consent to sexual conduct. It doesn't matter what she says or what he says either! Now because he is 17 remind him that he is now in the eyes of the law an adult and can be treated like one! he could gain a record for Statutory rape and this will stay with him forever!
Yes that is right 17, just to give you an example of how this could affect you- get caught with a small amount of marijuana at 16, you get a caution that will disappear (I believe after 12 months), however do the same on your 17th birthday and that caution will never go away! It ain't worth it! not a lot of people our age know that.
do not let someone who is not able to make rational decisions make their own mistakes, If anything happens you will hold yourself responsible, that is bad enough! the stakes are incomprehensibly high so stopping it is the only safe bet.
I hope this has helped, I think your sister will appreciate that you care, not now, but you are going to have to live with that. Just because it went ok for some of the people above doesn't mean it will go ok for her, or make it any better.
Good luck