I think fallenninja has replied on my behalf really well! I think it's a totally different scenario because it's a parent/child relationship we're talking about here.
My only exception to that is dd's Dad, and I do buy him a gift at Christmas from the girls, and have welcomed him into my home - purely for the girls' sake.
I think that's a really good point about children of separated parents being able to up sticks and go to, or threaten to go to the other one, if they aren't getting their own way. It was something dsd often used to do, and I would say to xh that if he and her mum were together she'd have to be staying put and sorting things out with them.
It does sound like the changing boundaries (or actually putting some in place) have maybe been the catalyst, but other things have added to that which makes it harder to deal with. The thing with the boundaries though is that she does need them, and while she kicks off against them, it is actually a way of showing her love.
My dsd, now older, says that her mum would let her get away with anything, as would her dad - and that if she whined enough they would give in. She tells people that she doesn't get away with it with me - and when she's with me I don't have the problems they have with her, and I feel I have the best relationship with her.
I honestly don't want that to come across as smug and like I have always had it sorted - I really haven't. She has had me in tears, said some terrible things to me, we have had stand up rows, I have at times been grateful for the times when she wanted to stay at her mums, and then there was the issue of me being the bad guy because xh wouldn't enforce any boundaries or sanctions.
I really hope that as she matures your dsd also starts to understand what has been going on. It really isn't easy for you, and especially for you as the step-mum.