Issues with Adoptive Mom over finding Birth Mom

Ok here goes. About 8 months ago we got word from my husbands ex wife that she had gotten information regarding his birth mom. She then forwarded us the info and my husband thought about it and then contacted the agency. Over the course of a couple of months letters and emails were exchanged and then permission given by us for phone calls and visiting. My husbands birth mom has turned out to be a really wonderful person who went thru much heartache in giving up her son. She was forced by her father to give him up. She gave birth to him and then they were moved to the red cross facility for unwed moms where she proceeded to care for him for a week. The day she signed the papers her sister picked her up and whisked her off to the hospital to see her dad who ended up dying that evening. She went back to get her son, but it was too late and the red cross would not give her the info where he was. Closed book! When he turned 18 she started her search for him. It took another 23 years before she found him. She had been told that her file had been lost. All of a sudden she received a phone call saying that they had found him. The file had been found and that was why they were calling. So birth mom and son are reunited. Robert found out he had a half sister and that there are other relatives. Now I being the friendly person that I am posted a note on facebook saying "welcome to the family" and my husbands family saw that and got irrate. I apologized and explained that we were taught to make people feel like family and comfortable. I did not mean any harm. They felt that I was giving this woman status and rank in the family and that was not my intent. The mom and birthmom have talked a couple of times and mom sent pictures and a long letter, etc. The birthmom was thrilled and felt like she was beginning a relationship with mom as well and very excited. There have been a couple of flair ups over information ommitted or you talk to them more then us... etc etc. Then I apparently fopared the other day by writing on his half sisters wall and saying mom instead of your mom (I write, like I talk). Needless to say everything went boom again. Birth mom was on her way to the city mom lives in for a convention and they were supposed to meet. Mom cancelled meeting. Birth mom emailed and begged for her to meet. They did get together for a couple of hours and birthmom thought it went well (although she seemed guarded about the info she gave us) and mom didn't care for the meeting at all. Never has birth mom tried to claim mom's position or ask Robert to call her mom. As far as I'm concerned he got two great ladies in his life. But mom will always be mom. So trying to understand why in all of this. Can someone give me a clue. We have no desire to hurt anyone and are enjoying the new relationship, but not throwing out the old for the new. I'm trying to read up on this, but sometimes it's better to hear perspective then to read all the journals. Thank you for whoever reads this and responds.