Purchase a copy of the book "And They Were Not Ashamed". I just bought a copy of this book the other day because I had some questions and issues about sex. It has helped me out immensely. The book is available thru Deseret Book online.
The book starts out with the "Good Girl Syndrom", I think this a lot of the issue with your SIL. The author goes into a lot of detail on how girls and boys are conditioned through parents, the church, and society. As I see it, the church needs to start a class for parents on how to appropriately teach their children abstinance but also teach them how sex is ordained of God, how sacred it is in a marriage to bring the husband and wife closer together. IT IS NOT DIRTY AND A SIN!