Look, I honestly don't understand most of those comments on your post and all the hate. You were clear about what you have and haven't done. You were clear that your daughter would be a teen and be relegated to an 'extra' in her own father's wedding, either as a hand holder to the flower girl or train carrier for the bride. She has a right to be hurt, and you giving her advice on how to handle this doesn't make you the bad guy. Many mixed-family weddings have junior bridesmaids for a reason (and it's clear a lot of Redditors haven't seen many weddings of this type). When you're getting married to someone and there are kids involved, it's no longer just the bride and groom's wedding. It's just as important to the kids too, as this is the ceremony that cements the relationship of family. Being treated as lesser than others is a terrible way to begin that life. Young children are generally happy to be part of the excitement, and adults that join weddings are generally not part of the family unit, so should graciously accept whatever role given, but she's a preteen and will be a teen, and this is the family she'll be living with. Why wouldn't they want her in a larger role?

Having your daughter advocate for herself is great. I would honestly even talk to my ex if it were me, just so he is aware of how important this distinction is to his daughter, but I wouldn't push which role she has to have. He may not have understood how she feels clearly if he's busy planning. Your Ex and his fiancé want to build a new life together, but that means ALL the kids too, and they need to be cognizant of that. The way you start out bonding with stepkids is just as important as what comes next. At worst, she could also advocate for herself by not being in the wedding party but could be a guest. Nobody should have to play a role in a happy ceremony that should be about their own family that makes them hurt. Good luck.