I think it's great that she has confided in you! 15 may seem young but it doesn't suprise me at all. I went to a girls school and there was so much pressure to lose your virginity and "pick up" at parties every weekend
However I was terrified of losing it! My first time was with my DP when I was 17 and we had been together in a serious relationship for about 3 months (we had "liked" each other for much longer ) and I'm so happy I waited! I have to admit, I was very anxious and worried he would be getting impatient with me

Anyway! If I have girls, I would hope that they find me approachable enough to discuss these things with and confide in me like your SIL has. I would be very proud too
Gawd I made DF wait almost 12 months before we DTD ><;; I was 17 and he had just turned 18 and we both lost it to each other. Poor boy!
I must admit alot of my friends now asked me if I feel like I've missed out being with one person and if I ever wish I'd "played the field" a little more and I honestly feel that I haven't missed out on a thing.

It's great she can confide in you, I think. Gosh I do feel like I'm starting to get a little old reading all this! I can't even think back to what I knew when I was 12 but I'm pretty sure boys had cooty's still XD