ETA: Brilliant answer, Sarah & Robert!
Original answer: I am a stepmother, and I would never have considered jumping in on my stepdaughter's mother's role in her wedding. A friend's daughter is getting married, and the ex-husband has been a pain about what he'll help with and not help with. It's made it so hard for the daughter. I suggest that you do as much as you can to disengage and take the emotion out of it. Not that it's not emotional - it is! - but you need to be clear-thinking and not turn your hurt feelings into a stressful time for you and your daughter. There's enough of that being done by the stepmother.
I suggest that you give them firm boundaries. Say that you are paying for the wedding and that, even if you weren't, this would be a special mother-daughter time and you know that they will respect that. If you can think of something that WOULD be helpful and relatively difficult for them to screw up, can you ask them if they would like to be responsible for X or Y? For example, if you and your daughter are not entirely wrapped up or devoted to finding music for the wedding or reception, would it be possible for the ex and his wife to research DJs or string quartets or soloists? What about photography? Maybe she can research photographers, although if you are paying, you want to be in charge of telling the photographer what pics you want and what you don't want. What about researching possible bridesmaids' gifts (ask for 10 suggestions, from which your daughter MAY choose.)
Tell them that you are handling the wedding dress and the flowers and the invitations, and whatever other things you absolutely feel you must (or want to) handle on your own and with your daughter.
When my stepdaughter got married, I gave her a shower (to which I invited her mother), and I was invited to a huge shower given by the mother's sister where I knew no one. But I held my head high, went in, and was pleasant but not the center of attention or expecting recognition in any way.
Perhaps the stepmother could host a shower for your daughter for your ex's side of the family and any friends that your daughter knows or who might care about the daughter/stepdaughter of your ex and his wife. That could only benefit your daughter by making her the center of attention and it could benefit the stepmother by giving her something to do where she can "shine", but which has zero impact on your end of things. As the mother of the bride, you shouldn't give her a shower yourself anyway. Your side of the family, the groom's side and your husband's side can attend a shower given by a friend or one of the bridesmaids.
When my stepdaughter got married, she gave me a small bouquet and had me escorted by my seat by one of the ushers, then after me came the mother of the groom and finally her own mother. I was very touched that she included me in this way. Perhaps some sort of small honor like that would be something your daughter could give to her stepmother without sacrificing anything. I think it will make you look like a magnanimous person to smile through it all. Everyone who knows you - everyone who matters - knows that your daughter spends little time with her father and his wife, and that you are her rock. The more you can show you are confident in your situation, the better off you will be.
Perhaps you and your daughter can think of some special way to acknowledge the role of your husband, her stepfather, as well. That way, the "steps" are included, but given a special role to fill, and limited from other areas. Perhaps, if there is a father-daughter dance, there could also be a stepfather-stepdaughter dance. If that's not appropriate, perhaps you can think of something else.
I read a funny story about a mother of the bride and a pushy stepmother. The mother of the bride bought a dress for the wedding, and then the stepmother bought the exact same thing, and refused to take it back. So, rather than start a fight over it, the mother said, "Alright, if it's that important to you, Stepmom, you can wear the dress you chose to the wedding, and I will wear something else." The daughter was baffled: "Mom, you LOVE that dress and you look fabulous in it. Why would you give it up?" The mother calmly replied, "Don't worry about it, dear. I'll just wear my dress to the rehearsal dinner the night before!" I love that story!
I think, if you give the stepmother/father a very small and contained role, and then very calmly thank them for any other offers but say, "Thanks anyway, but we've got that covered," you'll do better than if you get sucked into the drama. Maybe your ex and his wife are clueless, maybe they are trying to make up for years of being disengaged, maybe they are embarrassed that they have had so little role in your daughter's life. Assuming it has been their choice to stay in the background (rather than you keeping them away), they're kind of stuck with their situation now. (FYI My stepdaughters' mother did not include the 2 girls in her 2nd wedding either - they got to stand at city hall while the marriage was performed, but they didn't get to wear anything special, and they weren't invited to the reception! Still, when one daughter got married, she took the high road with her mom.)
And give your daughter the vocabulary to set boundaries. If the stepmother says she wants to help with the dress, tell her to say "Thanks, but that's something I am doing with my mother. I'm sure you understand." Then she should change the subject.
I'm sorry for what happened with your sister. My husband's ex did something similar when my husband's mother was in the hospital and it was awful. What we learned to do was not to give any info, and to write orders that certain people couldn't be admitted to the room or allowed any decision-making role. (She positioned herself as "the daughter-in-law" and so the staff sometimes thought she was my husband's wife and very involved, although she had never like my MIL at all.)
For your own sake, I hope you can find a way to not take this personally. This is not about you. It is about her issues and her problems and her inadequacies. My brother's fiancee's mother declared ahead of time that she planned to stand up in the middle of the wedding and slap my mother! So my parents alerted the clergy member and church staff, and then went forward with their own roles as assigned by bride and groom. They didn't let it ruin their day.
You have to take charge of things and give instructions to anyone you hire (photographer, caterer, clergy member, etc.) that YOU are the person contracting for their services, no one else. That you are close with the groom's family is wonderful. Work out with them what's happening and who's doing what. If you are comfortable, share your concerns and stress about the stepmother, and ask their advice and support. This is a great place for your husband and the groom's father/mother to step up as a united front. That doesn't mean to start a fight, but simply to say that it's all under control.
Ultimately, your daughter's wedding day will be the day that you will shine, that you will be seen in your honored role. Have confidence in that. Do not let lesser people take this away from you. Pity them for their selfishness, and contain them. They are not in charge. Throw them a bone if you can in an area that doesn't matter that much to you, but otherwise develop a distance and a few select phrases which you can repeat as needed: "Thanks but this is between mom and daughter." "Thanks, but as I said previously, this is between mom and daughter."