When to meet birthfamily (non-open adoption)

Perhaps someone can help us.We adopted our daughter, the day after she was born, and we have had periodic contact with the birth mother, who we really like. She ended up marrying the birth father, and they now have two young children. Our daughter is now nine years old, and she has known she was adapted from the beginning. And we've always promised her that she would be able to meet her birth family when she was older, with the idea that when she was 18 she could make the decision as to when. We are now rethinking that decision, and she periodically mentions a "hole in her heart" or a feeling of loneliness that I believe is common to adopted children. We just heard from the birth mother again, and learned about her current family situation, which is very good. They are very willing to establish contact, but also respectful if that does not fit what's best for our daughter. Coincidentally, we found that they had moved and now live quite close to where our oldest daughter is going to college, so that setting up a meeting would be quite easy from the physical/travel/proximity standpoint. My gut feeling at this point is that the truth is always best. And that my daughter might benefit from meeting her birth family, especially since she has two full blood. Younger siblings. The cautious side of me, worries that this might cause problems for both families and especially for our daughter. Do any of you have experience with a situation like this, or have any advice? Do any of you know, professionals that we could consult that have experience with situations like this? Thank you for any input, you can provide .