But OP's eldest daughter will no longer be exposed to OP's in-laws treating her differently, or having to live with a father figure who treats her differently. I know the youngest daughter would still get stuff that the eldest doesn't, but the eldest wouldn't have to see/live with the people who are excluding her. I bet it would be a huge relief for her never to have to see the MIL or stepdad again.
And OP's husband does treat her differently, because he has not stopped his family from being mean to her. His daughter from the previous relationship (OP's stepdaughter) felt free to tell eldest daughter to go away when she sat down in her own home to watch a movie with them. MIL feels free to express that she can't act normally around her grandchildren with OP's eldest daughter there. (WTAF?? I don't think I've ever heard of anything so spiteful, and I've heard a lot of things.) And then we have the Disney thing and this hobby thing, with no effort from the adults to even things out, which OP's husband should have done.
From what OP has described, her eldest daughter is one hundred percent the Cinderella in this set-up, and it must be very damaging for her.
If I was OP, I'd have a huge urge to gather up my eldest and go off and have a nice life together. (Obviously with the youngest too, when OP has custody.)
I don't think I could get past the meanness and the utter disrespect shown to OP and her eldest. And her husband sounds completely wet, saying he didn't want to get married but then got married. Why didn't he stick to his guns, if that's how he felt? Why am I not shocked that the same man is too weak to insist that his stepdaughter is treated better?
OP's eldest daughter may not be a granddaughter of MIL, but she and MIL are connected via OP's youngest daughter, who carries both MIL's and the eldest's DNA. Put another way, OP's eldest is half-sister to MIL's granddaughter, so the eldest does matter in a familial sense. She's part of the tree, same as MIL.
I cannot believe that MIL took NINE children to Disneyland and yet there were no funds to include OP's eldest.
I think you're best off away from this family, OP, I really do.
I have no respect for people who are all MY BLOOD MY DNA ME ME ME because they often cannot see the value of others. And who do they think they are, anyway? Superman? Wonder Woman? What's so special about their DNA? 😡