Maybe you overindulge her because she is a lot older, and because it's possible she feels left out of the family unit (so to speak, not being like her sisters with her mum and dad), and so you spoil her.
If so, then she will constantly want more from you (to fill her love tanks and feel wanted) and she will push you to give more. If this is the case you have a problem because she is feeling unloved.
Maybe you don't overindulge her or don't get along side her at all and maybe she is genuinely feeling angry and feeling left out. And so you see her lash out.
If so you also have a problem.
The thing is, it would be very difficult to get into her world when you have three younger kids (who take up so much of yours and your husbands/partners attention - and not her dads).
I suggest this might be the problem.
She is only 9. I would assume she is still figuring out her new family.
Nine (9) can seem so much older when they are the first child, and so much younger when they are they are the youngest.
9 is still very young. Don't forget that.
I'd be surprised if she genuinely doesn't want to go to Disney. I assume she more likely just feels angry and hurt for some reason and again I would suggest she feels unloved.
Kids rarely feel this much hurt and anger and lash out without have seemingly valid words in their heads like "I'm unloved" or "I'm unloveable".
They also pick up on our unspoken words (and actions) more than our spoken words.
The oldest can also sometimes feel like they are expected to be more able to do things and more independent (self sufficient) before they feel ready for it.
Maybe she genuinely feels pushed further away from mama, because of your succeeding babies. It doesn't take much for kids to see things wrong or make assumptions (and they will get it wrong).
I suggest be really clear, kind and loving towards her and see how she goes. Take her out once a week (as the other posters say). Ask her where she wants to go. Let her feel like she gets to plan where to go. Let her enjoy the fact that Mama and her are going out and that Mama will pay her all the attention.
Get in her world.
Genuinely try and understand her more and what she likes and what she thinks about things (her opinions).
Let her teach you how to do something, like how to play a game or fix something or how to brush her dolls hair.
Let her be centre stage for once. I think you might see a different girl come out of her.
Please though, do not leave her at home and do not let her miss the holiday.