Then undoubtedly she would think your dress too white/cream/ivory.
Look you don't have to agree with us, or her, and you can think she's a total bridezilla. I'm also very sorry you're own wedding was cancelled, I can only imagine how devastating that would be.
I also think the dress you've chosen is fine for a wedding but - as you've seen - it's a touchy and personal subject.
However, your brothers fiance has heavily hinted that she would be upset & offended if you were to wear a dress like the one you've chosen, to the extent your brother keeps asking you to find something else.
The overriding opinion her is that you should be gracious and choose a different dress, whatever your personal feelings,because the wedding day is not about you, or your mum, it's about your brother and his wife, and it's the nice, kind thing to do.
I fully appreciate your time may be short but it's totally doable to find something else. In fact, in the time length of time this thread has been running you could have found and ordered an alternative dress.
I understand you're feeling attacked, that really isn't anyone's attention. You can avoid the whole drama, and a lot of hurt, if you just do the gracious thing and find a different dress.
I really don't understand why you wouldn't?