Ok, I've been following this thread for a while now and not commented as have been waiting for OP to provide more information.
I've written the comment below based on the fact that I do sympathise with the stepchildren but someone has to think of the OP in all this.
These kids HAVE (whatever anyone says) been provided for in their father's house. Surely everyone should be considered in this. I think the OP has done her absolute best to make the situation as easy for all of the children as possible.
She has (from what I have read so far):
Accepted the children into the home
Treated them exactly the same as her biological child
Given these stepchildren the choice over where to sleep
Given up her BEDROOM to accommodate them - no one seems to acknowledge that this is actually quite a sacrifice.
Done her best to get along with the children, taken them out etc.
All of the above surely means that she does not deserve the pasting she has received on this thread and I am angry for her. Does marrying a man with children mean that you have to put up with unreasonable demands and have your needs disregarded?
Also, I've noted some of the comments that seem to justify the ungrateful behaviour of these children because the DP treats OP's little girl like his own and wonder if this is why stepchildren feel resentment. Is it not expected that stepmothers treat stepchildren like their own? Why not stepfather's - or is it only stepmothers who are supposed to give themselves up completely?
No one ever seems to say:
"What about the biological children effectively sharing their parent with children not related to them?"
Fair enough, if the children have been or are being treated badly at their father's house then they have every right not be abused and should be listened to.
However, there is no evidence of this. All I see is a woman who has bent over backwards and given up her space, time and home to provide for her husbands children who DO actually have their own home and bedrooms presumably.
Surely both bio parents of these children should be putting their foot down and laying down the law at this stage (except in cases of abuse) as no child should have the power to disrupt family life in this way.