Wow reading a few posts you are getting a hard time OP, to me unfairly so.
Of course it’s going to bloody hurt, step mum and Dad swooping in and commandeering this special event. They are being the assholes here. They’ve let you do all the hard work over the years, and are being selfish and putting your DD in this position.
Maybe it’s because you said you wanted to tell your DD to sod off, and many posters are referring to their own experiences of feeling guilt put on them and not cared for.
I don’t agree with posters saying that kids will naturally get it, see which parent really cared and which bought them gifts instead. Plenty of people don’t appreciate their parents! Plenty learn to manipulate Daddy and learn that money equals love. Or that it’s ok to be irresponsible because you get the better rewards.
We must not put bitterness onto them though. I agree with posters there.
However staying silent is being a martyr. And the best way to stop being a martyr is to speak up, assert yourself, and ask for first consideration from your child. Yes she’s been put in an awkward position- but by her Dad and step mum, not you.
Although I actually love the suggestion of choosing the dress and then sending DD to her Dads to buy it! It’s the shared moments you want isnt it OP? Nothing at all wrong with asking for that. Try not to be angry though!