I get on with his ex BUT....

I am in a relationship with a great man. His ex-wife and I have become friends. My problem is that his family do not understand this. When he met her, she had two small children who he raised with her for 7 years. Their relationship unfortunately broke down after they had a stillborn child together. He still visits her children as he considers them his own after raising them for many years. This, his family find difficult to understand as "they are not his children" As he visits the children often, I have come to think of them as my step-children. Because of this, I felt it important to build a relationship with their mother. Over time we have become really close friends who are able to turn to each other in times of need. I find it refreshing and wonderful that we are all on friendly terms and feel that this is best for the children also. The problem I have has arisen from the fact that he and I have just had a child. Because of the close relationship between us all, I know the devastation the birth of their stillborn son had on =my partner and his ex. Partly due to this and also due to our friendship, we decided to ask his ex to be godmother to our little girl. This has caused quite a commotion. Over the past week, both of his siblings have informed us that they will not be attending the christening as they find the situation 'bizarre'. My feeling is that this is a slight on my child and that if they cannot put their personal feelings aside and join in our daughters special day, that they are effectively saying that my daughter is less important to them than their objections to my partners ex. I am so angry at the moment that I feel like cutting them out of my daughters life completely. My partner and his ex separated and divorced by mutual consent. There was no blame on either side. The split was ammicable on both sides. So why oh why do his siblings persist in a vendetta against his ex. I would appreciate anyones thoughts on this and any ideas as to how the situation can be resolved because as you can imagine, this is very stressful and upsetting for both myself and my partner.