I was married at 16 and first baby at 17. TALK TO HER, just make sure you tell the truth... 15 year olds are very rebellious and/or confused... doesnt matter who talks. She needs to understand the most important thing, in my opinion, is that she is careful, because odds are, soon she may be doing it.

I believe you should tell her what she wants to know better for her to find out through you then go experiment with the wrong people and find out for herself. Sex is such a natural subject I talk to my cousins all the time about sex and they are very open-minded and actually talk back to me about their experiences. I am a firm believer that sex is not a subject to be ashamed about or immature about either. The more confidence she has with you the more your going to know what goes on in her life and your sister will appreciate this in the future. Besides that being said my cousin is now 17 we started talking to her about sex since she was like 8 she was very curious were baby come from.

Part of the problem with this subject is that people seem to be afraid to talk about sex. My mom talked me about it and stressed that I was too young and maybe went a little far by telling me that a man's penis is too big for my body. But, I didn't have sex until I was 17 and it was with a guy that I was with for 2 years and almost married later on and I was truly in love with him. Also, I was taught sex ed in school but, they don't do that anymore either.

I think that if your sister is not willing to talk with her daughter about sex then you should. I am surprised that she hasn't already gotten the info from her peers. You should feel proud that she is willing to come to you and ask those type of questions. Usually at that age they are more willing to talk to their friends then 'yucky' adults.

I just realized my answer is a little late but, I think it is good advice...