D.B.
I am a stepmom to 2 daughters, and a mom to 1 son with my husband. I understand the challenges.
Here are my thoughts:
1) This is an insecure stepmom who needs to buy your daughter's affection and loyalty. Your kids don't want to go there, and you allow them to stay with you, which makes the stepmom try even harder. She feels rejected by your kids and she's trying to buy loyalty and affection in any way she can.
2) Your daughter is getting ready to leave home. It's totally normal in ALL families for kids to start to make the break now - it makes it easier for them to do what's so scary, which is to go off to college. It's easier for your daughter to give in to this woman and say you are the bully, because she is trying to bide her time until she leaves. If the stepmom is relentless, she's wearing your daughter down.
3) It's impossible for another person to destroy a healthy relationship. A 3rd party doesn't break up a healthy marriage, and a new stepmother doesn't trash a healthy mother/daughter relationship. You have to stop appearing to be as needy as the stepmother is.
4) Your daughter isn't engaged and doesn't have a wedding date. Why are you sweating out the purchase of a wedding gown that isn't even on the horizon? Let it drop and the whole thing will go away. Stop thinking about your "rights" and think about your daughter's predicament. If and when she gets married, it will be her wedding and your job will be to support her. She will see, either through the etiquette books or the wedding planner advice or the brides' magazines what a role her mom should have. And besides, I helped a friend buy her wedding gift because the mom couldn't come, and the mom still had a huge role and a place of honor as the Queen Bee at the wedding! But you're not there yet, and you're getting upset because of a silly competition. Don't. Your daughter needs a role model, and the needy, bribing stepmother is not it. Let the whole thing die down - you'll be glad you did.
5) Your daughter and you are NOT best friends! At least, I hope not! You are her mother, and she needs you in that role. She does not need you to be someone she takes care of. You need to show her that you are strong, confident and independent. You both need best friends in your own age groups. The surest way to push her away is to be so needy that you become her "job"! My stepdaughters' mother is like this, and it's just awful. They are afraid to do anything without taking care of her first, and it has turned them into dependent young women who don't know how to stand up on their own two feet, even against the men in their lives. They are in abusive relationships because they think women are weak and there to get beat up on.
6) Your daughter will miss you a lot when she goes to college, and she will appreciate all you have done for her when you aren't there every day to do it. She might not tell you this, but you will see it in her eyes, in her attitude, when she comes home at Thanksgiving or winter break. Let it dawn on her gradually and don't keep picking at her to tell you how much she misses you. Be patient.
7) Send your kids to see their father and stepmother. Don't turn this into a competition or show that you are jealous of her. Insist that they have a relationship with their father, whom you seem to think is a good guy in many ways. Respect the woman he married, hold your head high with your position as mother, and encourage the kids to have a relationship with their step siblings. One day your daughter may get married and it will be truly awful if there is dissension or a bunch of adults she has to worry about.
8) Take the high road here. Show that you are strong, not petty. Repair your relationship with your daughter if it needs it, but don't blame it on the stepmother. See #3 above. Whatever problem there is, the stepmother didn't cause it.
9) Have a life beyond your kids. Do some adult fun things, with girlfriends or maybe a date! Show your kids that you are a full and complete woman, not just a mother. That is the vision they need to have in their heads.