My ex SIL (we hated each other) pulled a stunt at my wedding. l only had one bridesmaid, my 22 year old cousin. My wedding, my choice.
A few weeks before the wedding, SIL announces that her mum's neighbour has "some gorgeous pre-war silk" which she had offered to make her 1 year old a dress for the wedding. "And l hope you don't mind, but it's peach (the colour l had for my bridesmaid!), l hope you won't think we're trying to make her look like a little bridesmaid!" sickly sly grin
Which is exactly what she was trying to do. She said it in front of her parents and my in laws to be, knowing that if l refused l'd look like a complete bitch. l was furious, but couldn't do anything about it, and on the wedding day, if anyone commented about "the tiny bridesmaid" l put them straight.
Please don't do it. Or ask the bride, but you risk the situation l was put in. People don't like to say no, but it could spoil her day.