I just found a copy of this off ebay. It's a kids picture book (about third grade level) about a girl who tells how her twin brother wound up as a bridesmaid at their aunt's wedding. The kids were English, but the aunt lived in Australia. She never realized that 'Jo' was a boy and when she got married she asked the sister to be a bridesmaid. The family didn't realize that Jo was included in that invitation. In order to not ruin the wedding, Jo (who is about ten) is forced to play the girl during the ceremony (the aunt just thinks Jo is tomboyish, and never finds out the truth). Later, Jo is embarrassed by a clipping of the wedding in the Australian paper, describing the 'sisters' as English roses.

It's a silly kids story, but really reads like some of the TG fiction we love. A nice read if you can find it.

https://www.amazon.co.uk/Lighthouse-White-Level-Brother-Brid...