It sounds like this woman is pretty unsympathetic--I wouldn't want to be stuck in a car with her to begin with.
This may end up being a hassle, but most churches have rooms where the bridal party can change for the wedding. A lot of them have classrooms for Sunday School or whatever that will let you change in there. I would be willing to bet the bride is not trying to ride in a car in her dress either. At the very least, you could change in the bathroom and have someone guard the door. I would ask at rehearsal if the church will have a changing area and then you could take your dress in some kind of bag (a garbage bag would work in a pinch).
Hope this helps--have fun and don't worry about mean people!
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