Grandmother being stubborn about wearing a bra with her formal dress

Hey all,
I'm getting married this April and my grandmother is looking for a dress. The style she's leaning towards is a floor length mother of the bride dress with silk and bead work. The one she likes is beautiful, however when I asked her what undergarments she was planning on wearing I was really shocked when she said she was not going to wear a bra. My grandmother is a size 14, and has breasts that on the larger side. Also they hang down to her belly button, as they do when they get older. I don't care if she doesn't want to wear a bra every other day off the year, but the silk dress with the beaded neckline that she chose looks very bad without something under it. We're a close family, she's going to be in our wedding portraits. I tried to compromise and lead her to the dresses that have jackets, but she's insisting on the one where they will be most noticably saggy looking.

I'm trying to tell myself that it's not disrespectful for her to not wear appropriate undergarments at her granddaughters wedding, but it's really frustrating me. If she was the type to not believe in bras, or if she was infirm that would be one thing, but she keeps saying "I'm 85 years old, no one will care, bras are uncomfortable."

Yes, formal-wear is uncomfortable. We're not going to a pajama party, it's a wedding. Her only grandchild. The most special of occasions. I don't want to look at all my pictures and try not to notice my grandmothers breasts. What do I do here?