I could understand if she was a little insecure about the wedding. I mean, you two getting married might call into question her role in your lives ... but not even able to talk about it in front of her? That smacks of cowgirl to me.
Speaking as someone recently married, my boyfriend was completely supportive of hubby and me. For our part, we reassured him that his role in our lives would not be changed just because of a party, a piece of paper, and two shiny metal rings.
Boyrfriend wanted to be at the wedding because it was an important day for hubby and me and therefore, important to him.