Should I say something to a guest that is planning on wearing a formal dress to a wedding she has been told is casual?

Edited because post title is confusing. Wedding isn't 'casual' (the suggested attire is 'beach elegant' and explained below). However, her chosen gown is wayyyy more formal than our wedding and more formal than my bridal gown!!

Hey Weddit!

My fiancé and I have a beach wedding coming up in November. We have a wedding website where the suggested attire states 'beach elegant' followed by "Suggestions: For the ladies, a long or short summery cocktail dress, a dressy skirt & top, or nice pants with a blouse would be perfect. For the men, a summer or linen suit would fit right in, as would nice slacks with a collared shirt. We don’t want to sway your swag, but please stay away from shorts, casual t-shirts/crop-tops, swimwear, etc, please. :)"

My fiancé's sister in law (the grooms brothers fiancé) has stated that she is looking for a dress to wear to the wedding, and at one point said her budget is 'up to $1000'. She has said she wants her dress to be 'formal, bright, and sparkly because her soul is made of glitter'. She has sent me pictures of dresses that she is interested in trying on and the ones she likes are literally PROM dresses and seriously blinged out. (Think hot pink prom gown covered in crystals).

I don't want to be a controlling Bridezilla, but she is going to be part of the processional and also in family pictures, and to be honest, I don't want her wearing something that is more formal that my own wedding dress. I have tried casually saying things to her like 'it really isn't a very formal wedding, casual is just fine' and 'you're crazy haha. My dress isn't even that fancy or sparkly'. She just doesn't seem to get it and said 'I'm sure your dress is pretty. I just don't really do casual for weddings anymore, especially family weddings'

We recently got in a small non wedding related argument (I said something that wasn't directed at her or about her and she took it very personally) so I feel like I am walking on thin ice with her right now. Is there anything I can say to her about wearing this type of outfit to my wedding without pissing her off further, causing family drama, and without coming across as a complete bitch? Or is asking her to wear something more casual overstepping things?

Thanks in advance!

TL;DR: future sister in law is part of the wedding processional and family wedding photos for our beach wedding, but wants to wear a hot pink, jewel encrusted prom dress that is fancier than my bridal gown. Not sure if I can/should say something to her without sounding like a bitch.

Edit 2: thanks for all of your feedback. It helped me realize that I just need to take a deep breath and let it go. Hopefully when she is trying on dresses she will realize that what she wants to wear is a little ridiculous for this setting. And if not, I'm sure that everybody else will realize it when the wedding comes.