When we moved in together which was DMs idea she said we would pay half each for all bills, even though I had 4 DC after a couple of years she said she should only pay 1/4 as there's only 2 of them!
So she underestimated how much half the bills for a household of 8 would be. Irritating, yes. But doesn't necessarily mean she's impoverished. When she said this you had a choice - either stay and accept those terms or both of you find different living arrangements. You chose to stay.
She's always paid for holidays just her, me and 3 DC because she's always had savings and we haven't, D H is out of work a lot!
So this isn't an issue about you being worried about her finances, if it was you would turn down her offers to pa for your holidays. It's simply that you don't like her paying for her niece instead.
Next year I wanted to go eurodisney for DDs birthday but DM said she wouldn't like that, her holidays are sitting on the beach all day and DC are fed up with that
If she's paying for the holiday then of course she gets to choose where you go. If you want to go somewhere different you need to pay for it.
Honestly OP the first post just made you sound unreasonable and with a bit of a chip on your shoulder about your DM spending her money on other people. After all the drip feeds you actually sound manipulative. It now sounds as though you have declined going on holiday with her this year as you want to go to Eurodisney and you were hoping that if you declined she would agree to that being the holiday instead and pay for it. Instead she has called your bluff and asked someone else to holiday with her.
Either way, your DM is clearly someone who likes company on holiday and is prepared to pay for family/friends to come with her to make sure she gets that. She gave you first refusal (which she isn't obliged to) and when you declined she made the same offer to someone else.