I would also suggest counseling - what Christine S. mentioned about displaced birth order is really important. We went through a much smaller transition a couple of years ago and the ramifications were much larger than we anticipated. My oldest son and stepdaugther are the same age (15), same grade but she is 4 months older than he is. They've known each other since they were both 3 and we got married when they were 5, but she lived with her mom and spent weekends with us. My husband and I have 2 kids together, boys who are now 7 & 9.
Anyway...my SD moved in with us full time when she was 13. Suddenly we had two "oldest" children - my son, who had been the oldest in our house for many years, and now my SD, who was simultaneously the oldest but the newest. Because the oldest kids had been friends for 10 years, step-siblings for 8 and roommates for 2 out of 7 days a week for years we didn't think the transition would be hard but it really was. My oldest son felt displaced, my SD felt like an extended-stay guest instead of part of the immediate family and as a result, we had some behavioral issues with all 4 kids. Some work with a family counselor worked wonders with helping us to recognize the dynamic and the tensions that the change in living arrangements caused. The end result was that we saw improved behavior and stronger relationships among all of the kids after a few months of counseling.
I would check with the adoption agency to see if they have any referrals to family counselors who are experienced with dealing with this kind of situation, which must be fairly common when adding to a family through adoption. Good luck to you, and belated Happy Mother's Day to you and your wife!