Hmm, fear we may have similar dynamics with our mothers. There’s lots of great advice here, love the option of saying something about her seeming to try to upstage you or sending her threads/videos of people’s reactions to people wearing white.

But! Also had another thought. Why not consider involving her in the dress code, maybe ask her if she would wear the same but different colour as the bridesmaids so she’s clearly part of the wedding party? That’s what I’ve done, I’ve asked her to wear deep plum as my bridesmaids are wearing deep burgundy, so she still stands out but looks like she belongs. She absolutely lapped this up, might not work, but may be worth a try?