Ok. Droves, I'm sending you a hug....make that several, you can put a couple away for future use. I think you're going to need 'em.
Firstly, stop with all the putting yourself last, thing. Today. You are important, or at least equally as important as anyone else in this family. Your step daughter has, for her own reasons, decided that her wedding is to be just parents. It's a narrow minded perspective when there are at least, 3 parents in this scenario but, perhaps she doesn't have the emotional maturity to see that. As time (and life) goes by, and I don't wish her any ill but she may have experiences herself that will bring her to the conclusion that, in this instance, she has been unkind verging on cruel in her inability to include family members who would love to be there on her special day.
Is it her? Is it her mum, not wanting you there? Is she simply and manipulatively making her dad "choose" between you two? She's the only one can answer that. Whatever. Her dad will be there. He might want to make a stand?
My DH, who's attending his daughter's (my SD) graduation this Sept WITH ME his wife, has been told that, should he attend with me, his exW won't go. He has said, if she so chooses, that's her problem and very sad. (background: His ExW's affair ended their marriage 4 yrs before he met me.) But, he has said WE will be there, come what may.
You know, let your SD do as she pleases. It's HER day. And you, do something lovely yourself that day. You and your twins. You sound lovely and she may come to regret this, in her future.