I've read this post and all your replies so far and I'm really trying to be on your side here but struggling, here's my thoughts, and why I'm struggling with it.
I'd never dream of dictating to someone what the can and can't wear to my wedding- so is your sister in law doing this to be awkward or is she genuinely concerned? It sounds like she's concerned and if it were me, I'd not want to cause upset.
Even though I wouldn't want to tell people what they should wear, I wouldn't wear white, or cream or beige, or champagne, vanilla to someone else's wedding- with or without a pattern.
You didn't reveal the dress until quite far down and kept repeating "it's not white". So I had in mind a fully floral dress which maybe had white border around a few flowers or a shadow or something.
But having seen the dress, it is a white (or cream) dress with flowers on it. I'm sorry.
You said that SiL said that it was a "similar" dress she'd seen - so maybe more floral? Less? Either way, it must also be a white/cream/ivory dress with s floral print
I totally understand that you've become invested in your chosen outfit and this may now seem a very inconvenient situation to be in, I would personally never.have dreamt of telling someone what they can and can't wear but there's obviously an issue somewhere.
Do you really want to risk falling out with your brother and his wife? said she's his girlfriend, but let's face it - they'll be married soon. If you agree with the opinion or not, it's obviously important to your brother and his family...
My best advice to you right now would be to have a look for something else where you can use the accessories you already have - and I say this kindly to you, don't be falling out with family, even if it's something you yourself would have no issues with. Your SIL hasn't seen the dress! So wear another - and they'll be none the wiser.
Enjoy the wedding! X