Sister in law

View Quoted Comment So glad you have people backing you up. I think that’s most important. I wouldn’t give her the satisfaction of knowing it’s bothering me. People are so weird and terrible at times lol

I would be firm and straight and simply send her a message and say you don't want to limit what she wears to the wedding but your only request is to not wear a dress that is basically white, and that that cream is just too close to white. Tell her you are totally fine with her dress, but you really want to not have something so light and close to white on your wedding day. Honestly is the only thing that's going to work here clearly, otherwise sounds like she's going to wear it and you're going to be upset.

That's absolutely disgusting of your sister in law. Seems to me she is being Envious and Spiteful. I would tell her about herself!

That's generally considered unacceptable guests will likely make rude comments as well.
Bring the spare dress and have her brother handle it
Sounds like something my sister in law would do. They're spoiled and spiteful for no reason.

Yikes, hopefully she realizes how narcissistic she is being. If she does end up wearing it, don't stress it. Everyone will look at her as the girl begging for attention and that's not a cute look.