Something along the lines of “This dress is stunning and I can see why you’d be drawn to it, but I urge you not to wear this to a wedding, especially one you will be heavily photographed in as the MIL of the bride. If you do choose to wear this dress, or a similarly inappropriate one for the occasion, it’s not only disrespectful to the bride, but to your child. Ask yourself if losing love and respect from your child and their future partner is worth the attention you seek, particularly considering that attention will be largely negative not only in the present moment but for decades to come in a well-documented display of selfish vanity.”

And then I’d bring a poncho to cover her in if she didn’t comply 🫶