When you say you're 'eloping' what do you actually mean? Because I took that to mean that it was literally just going to be you, your fiance and your daughters, with no other guests. If so, at what point is 'walking in the room' going to be an issue? Surely you just turn up as a family and go in together?
If you mean that you and your husband and daughters are just the bridal party and there will actually be a roomful of guests, that's not an elopement, and in that situation I can see why she would be nervous about being a bridesmaid or whatever. I feel like I need a lot more information about the actual wedding here and what's expected of her.
Anyway. If it's the latter, then I suspect she is just incredibly nervous about everyone looking at her. But if it's the former, then my guess is that subconsciously, she is probably actually worried that things will change after the wedding and is transferring her anxiety on to the ceremony itself. I doubt she's actually aware of this, but if her biological father was horrible and she loves your fiance, my guess is that there's a part of her that is secretly worried that your fiance might turn into someone like her dad once you get married.
Also, she's had little control over her life so far, from what you've said. Abusive father, lack of stability in the family, sudden arrival of a baby sister etc. So I'm guessing that there's a part of her that is just feeling like she doesn't have a lot of autonomy in her life, and she wants to remind you that she can make choices. She's testing you, I think. Trying to reassure herself that she can't be forced into anything, and also trying to remind you that her feelings are still important even on a day like this. Again, I strongly doubt she's even aware of this; it will all be subconscious.
If I were you, I'd sit down with her and say 'Look, Stepdad and I would love you to be there with us. We love you all and you girls are the most important thing in our lives, so of course we'll be absolutely gutted if you aren't there. This isn't just our wedding, it's a ceremony for us as a family. But if you really don't feel that you can be in the room, then we won't force you. Come to the register office and you can see how you feel on the day - if you really don't feel you can bear to come into the room, you can wait outside if you want and we won't make a big deal out of it. It would absolutely make our day if you could manage it, because it won't feel complete without you there with us - but we'll respect your choice to sit it out, if that's really how you feel.'