L.R.
Here's my parental strategy for many issues I have encountered with 5 daughters and 2 stepkids: the first thing you do is let the kid off the hook. Tell them they absolutely do not HAVE to do whatever it is. Then move on from there, telling them why they might want to. She will probably want to do it when she things about how much fun it might be, to look pretty and make people smile. You can put it in that context - how happy it will make the bride and other people for her to be a part of the wedding. But if she really doesn't want to I would not ever force it. It's just not important enough. I have been at a wedding where the ringbearer clearly did not want to be there, stomped down the aisle, threw the pillow at his mother who was a bridemaid and said, "Don't ever make me do this again." It was embarrassing for the mother, a jarring moment and definitely not charming in any way, not to mention being on video for posterity. Some kids love to be on show, some don't. Respect that.