Exactly this.
OP, if that’s truly what your dress looks like, it’ll be fine.
but for goodness sake don’t dress your little girl in white. She WILL look like a flower girl, and you’ll come across as someone trying to force her into the wedding party and people will be silently judging you.
My husband’s sister tried to pull that at our wedding. She was insisting her children all be part of MY bridal party, despite the fact she was on my husband’s side and I was very clear that I only wanted my sisters as bridesmaids and my niece as flower girl. Husband’s eldest sister had three children, their brother had four, and there was another much younger sister, not to mention all the girls on my side who were being pushed at me by cousins etc. It was far too much, hence why I kept it small.
Unfortunately my husband’s family thought they could control us and force the issue, to the extent that his older sister declared she’d dress her children as part of the bridal party anyway. Then Covid happened, our wedding was postponed, and we saw her listing actual bridesmaid dresses for sale that had been “bought for a family wedding but wouldn’t fit by the new date” on marketplace. Including a traditional white flower girl dress for her youngest, from a bridal company!
So, please don’t be that person. Wedding planning is stressful enough, and your brother and his stb wife are not going to look upon you favourably for dressing your daughter in white. Why don’t you choose a colour from your own dress and find one for her in that colour instead? That would look really cute.