you might want to steel yourself for the future...my brother married a girl 15 years ago who despised the entire family and I think my parents have seen them a total of 6 times since then, three of the visits to be with 2 little grandsons. She had a horrible family life and once she had my brother she decided he was hers and hers alone, she wasn't sharing, end of story. This - of course - did not apply to her own brothers and sisters, with whom they visited and vacationed from the beginning.
Truth be told, it would probably have gone better if my mother had been less clingy and more willing to release her children (she's still trying to re-loop those apron strings, and we're in our 40s, LOL!). My sister-in-law has finally decided after all these years that I'm no threat and there is a standing invite for me to visit any time, but talk about a rock and a hard place!
My father, BTW, responds to this situation exactly as your husband does.
Dear, I'm afraid the best you can do is to demonstrate to the girl that you are NOT a threat and wish only to visit with your son and future grandchild, not steal them away from her. Good luck to you, and I hope you'll be able to enjoy your grandchild when he/she arrives