Right. So you booked a lot cabin at Christmas, a bigger one to include her. The mother decided last minute she wanted to do something else. So you just paid for a no show, and that's fine
You've booked the Lake District. Mum says no, it's her nans birthday. You've still booked the bigger villa in case she can come. But if/when she doesn't, you've just paid for a no show. Which is fine
Except it's not. The mother is a problem. And the teenager is being taught it's ok to act like that.
On top of that, the teenager has been flaky and made excuses herself not to be included with things, and is allowed to cancel as she pleases.
Well. If that's the behaviour you want to go with, which is rude, disrespectful, and thoughtless of others, then don't be surprised when you aren't enabled anymore, because no one's dared to say boo to a goose so far, because it's the red carpet royalty that is a stepchild. Your own children don't get away with this. It's not down to you to accept shoddy behaviour and lose thousands of pounds of your money over theirs.
She's not attended the two examples above. She's (been allowed) to cancel other things. The mother pulls stunts. And this will cost you thousands of pounds which you only have because your family member has sadly died. But you let her and the mother act like this, because "poor darling stepchild". No. You don't get to behave like that, whoever you are.
And forget anyone from the first wives club pretending they'd be fine with it. They wouldn't spend a penny of that money. They just like to come on here with the stepmother bashing, probably because they've got one that stands up to them too, instead of letting them behave as they please, like you have been with the ex and her teenager up until now.
As before:
"Yours and DMs trip to Disney sounds so good, I think we're going to book and go too! It won't be as good as your trip, we can only go on the baby rides, you're so lucky! It's important to us you have the best time and we'd like to give you $200 (?) to really treat yourself, and take loads of pictures for us to see, we're going to be stuck in the little kids section and miss lots of the brilliant things you'll see, so please show us when you get back, we can't wait to see what you get up too!"
Then relax that, just for once you get to enjoy this once in a lifetime experience, that, unlike every other holiday, can't be affected by two other people's (deliberate) poor behaviour.