She DIDN'T "try to send a message through mum".
She didn't know that the dress she saw at your mum's house, that she said was too white, was actually YOUR dress.
She thought it was your mum's.
Hence she asked your mum not to wear white.
At the time they were discussing what your mum was going to wear to the wedding.
All perfectly normal, happens re thousands of weddings all across the country.
Following on from that, she and your DB sent out a message reminding guests about the 'nothing too white' dress code.
Again, perfectly normal for weddings these days. And in line with wedding etiquette.
If she knew that the dress she'd seen was actually yours, she would probably have specifically asked YOU not to wear white.
But as your devious mum "covered for you", she didn't know.
Your actual dress has more white than the one your SiL2B saw at your mum's, so you KNOW she will not want you to wear it.
But you clearly hate your SiL2B, as you have shown through all the things you have called her on your threads, so you are going to wear it whatever anyone says.
You seem gleeful at the idea that she will be upset at you wearing this dress.
I think she will chalk it up to her SiL being desperate for attention, trying to compensate for her own wedding being cancelled.
Or just sheer bloody-mindedness on your part.
There are certainly people involved in this wedding who have no class, and it's pretty clear who they are.