Meant to stroll by without comment, as I don't think OP needs any more negative opinions.
However, I've been puzzling for the last hour over whether the couple of Nan comments are genuine or sarcasm, as the image of either of my grandmothers even attempting anything like this is going to haunt my dreams tonight!?
In contrast, OP, you have the figure and youth to get away with this sort of style. Personally, I'm sure you can find something MUCH more flattering and appropriate, but if you think the bride wouldn't find it unsuitable for the venue/look she's chosen (ask her?), and you'd feel comfortable in it, go for it.
The fact you've asked though means you're not sure yourself, and could waste the wedding regretting it, especially as it's more of a 'red carpet strut' style, whereas during a long day of sudden activity, much hanging around, dancing, and meals and drinks at odd times, I think you'll constantly be pulling the straps up, the slit down, and the stomach in, rather than enjoying yourself.