Ok, I know I'm late and most everything I'm going to say has already been said, but...
I started dating my first serious BF when I had just turned 15, he was 17. Within a few months, we new we were going to be married one day. Our favorite dates were renting movies and watching them at my house, going to the movies and back to my house, ...
In the beginning, our parents were sceptical about our seriousness, but after a while, we became a part of each other's family. He had a sister a year younger than I and we were friends. (We met at our church youth group and had all of the same friends.) Near the end of our first year of dating, I stayed over at their house on New Year's Eve, because it was late and they didn't want him to have to drive the 30 min to my house to drop me off. I stayed with his sister. We had tons of fun and it became an annual event. As the years passed, I began to do more and more things with his family and he with mine. I was a little hurt when they would have "family" birthday celebrations, but in the long run, I learned a lot about families from his parents. They are great examples of parents and of a husband and wife (but that's a different thread). 8 years later, we are married, with our second child on the way. We are the ones who got married, then had the babies. We cherish all of the photos and memories of the times we had together before we were married, but we also cherish the times we had with our own families. I think it made our family stronger. The trips I took with his family were great, but I did have his sister. He never went on trips with my family, because it was just my parents and me.
Either way, if the relationship is truely mature, then she will understand your reasoning. Whatever YOUR decision is. But, being supportive of her relationship is great. Not too far one way or the other though.