It put a smile on my face to see a big brother who cares enough about his little sister to want her to make good choices in life. Sounds as if you're doing a lot already.
Ultimately, you're in a tough spot if you feel you have to be a brother and a pseudo-parent to her. It's a lot of weight to put on your shoulders.
If she's a "good kid" and you have a strong relationship, keeping the channels of communication open might be the best thing you can do. Way back in my teenage years, it was always harder to make a judgement call when I didn't have anyone I felt I could talk to. Some of the more "girl stuff" can be tough to talk about - both on the hearing and on the speaking sides - and if you're not comfortable, a good female role model (mom or guidance counsellor or cool aunt) is a more helpful option than trying to awkwardly fudge your way through tampons and diaphragms.
And being realistic is important - which it sounds like you're doing. Spelling out what you believe is appropriate (even though you'll love her no matter what she ends up doing) can go a long way when you're not around to be the angel on her shoulder. Or devil, whatever.
But, most of all, if you work with mom and her dad, rather than (intentionally or unintentionally) undermining them, the messages that she gets will be less confusing and (quite possibly) more effective. Working against them may see her get confused and more likely to rebel, end up with you cut off from her, or worse. As crazy as they may seem, allies are always more effective than enemies.
I won't see him for 3 months but hopefully after that we can go back to being friends. I think I might be over-optimistic but we'll see.
