I don't think you're unreasonable to want this at all. You're human, and it sounds hard work, infuriating, and completely unreasonable.
But.
She is eight. She's genuinely upset about this. Being left out of a holiday will just reinforce her fears. That's not going to help.
What will help is a preplanned line of attack from you and her father, with an absolute party line where you treat this exactly as if aimed at her sister with some cases, or her own mother if they were still married with another. So she interrupts? You ignore her and continue with the conversation as if she hasn't said anything, and if she persists, he sternly tells her interrupting is rude, to be quiet, and to wait her turn. She interrupts flirting teasing with "defences" of Daddy? Daddy asks her how old she is, and then if she really feels it is her place to interfere in an adult conversation? No? Then she had better consider carefully before she does it again, because she is being rude and it's unacceptable. She gets into a play fight you are having by staging a drama? With a weather eye on her actual welfare, obviously, you completely ignore her and carry on, and if she asks why, your DH bluntly tells her why. Calmly, factually and without accusation, but saying it's boring and silly to try to stop him having fun with other family members, and she knows how loved she is so he will not allow her to stop him loving other people, too. If she starts on, "you love her more than me!" he simply says that's too ridiculous to engage with and love doesn't work that way. There's enough for everyone - or does her loving her mummy mean she can't also love him? Does her loving you steal some of her love from him? Now stop being daft, because the conversation is over.
The thing is, this stuff is not exclusive to stepfamilies. It's common in all families. But when there is a stepfamily, suddenly the stakes for the adults are higher, the guilt is off the charts, and people stop remembering that actually, it's not only bad for little kids to get away with this crap, because as Coffee says they grow up repeating this successful strategy, they don't even want to get away with it, most of the time. It's the adults who are attributing masses of involved emotional baggage to it. From the kid's perspective, you're just someone her father loves very much. Plenty of mothers (and fathers) are resented by their own children on this basis, and almost all siblings. The problem is that we aren't equipped to cope, when a stepfamily, as adults. So we make a meal of it.
This child loves you. She loves being with you. She just wants to feel Dad loves her, and because she is both small and jealous she is trying to steal what she wants through manipulative means, because at some level, she is being indulged. Nice reassurance is worthless without firm discipline (and by that, I mean treating her rudeness as rudeness, and ignoring where you can, and otherwise being loving and normal with her). Dad is not going to make her feel better by seemingly not loving her enough to be her Dad and disciplining her fairly and reasonably - as long, of course, as he also plans special time with both girls, alone and together, where he makes it clear he loves her to bits (and, of course, continues to be loving to her in front of you).
Learning to share is hard. For everyone. This is being turned into a bigger drama than it needs to be because of a step situation. I've seen this so many times amongst my friends now, and it never fails to amaze me. She's just a kid being a kid - it's an effective, constructive, positive discipline requirement. No more, no less, IMO.