I could've written your post, I think I have actually...
I go through the same things with my step daughter, soon to be 7.
Her mother and I have actually chatted many times about behavior and she says that her child needs to be allowed to grow without restraints of any kind. She only needs to respect her mother and her father (her words, not mine) and now that she is in school her teachers. If someone gives her something as a present and she doesn't like it she is allowed to tell that person that their gift sucks and she hates it. So, in my instance, I do blame the biomom for teaching her daughter that it's alright to treat ppl with disrespect. Her mother has told her father and I that there are no rules, no guidelines at her house. She has the run of the house and can do as she pleases.
However, some of this is coming back to bite her in the behind as she and her husband now have a child of their own and SD is exhibiting the same behaviors there as she was here and her mom didn't believe us or want to help....and her husband, who has stayed in the shadows as a buddy is stepping forward now to tell SD that she isn't allowed to smack the baby, that she needs to listen to him etc....it's not flying bc he was a buddy buddy for 4 years.
A child only knows what it knows from it's parents, from what they see, how they are parented. It's so hard coming from a house with no boundries to a house with them (don't hit others, treat others with respect, say please etc...)...age appropriate ones.
My DH doesn't step up to the plate in these instances. He says he will talk to her about things and then goes behind closed doors and chats with her and then can't remember what he told her. If I come in, all talking stops and she glares at me (I knock first) and he sits idly by. And the times I have been "allowed" in on the converstation, I find he is chatting to her like she has the comprehension of a 16 year old, not an almost 7 year old. And you can't get anywhere if your child doesn't understand what you are saying.
I have been in SD's life since she was just barely 2 years old. It's not like I am new to her, I've been around most of her life. I've never tried to take her mother's place, she has a mother.
They have shared parenting...We get her every other weekend and mondays and tuesdays so at one time we will have her 5 days in a row, then her mom will have her. That has to be hard too but it's the way it's always bend and both parents are unwilling to bend about the visitation. DH would be happy to have her here 100% of the time to keep her out from under her mother's influence (I haven't delved into the eating disorders that her mom has and has passed down to her daughter) but I think if that happened we wouldn't be together any longer.
He wants the girls to share a room when we move next year so he can have a basement in the townhouse so he can go down there and do his thing with his things, a place to put HIS things (another entirely different post) and then let me deal with the disaster awaiting them sharing a room. I don't know how he thinks it would be good, despite the age difference, if DD so much as sets foot in SD's room or touches anything of hers SD goes ballistic and screams, yells, pushes, tantrums....but she will then come out later or another time and help herself to dd's things without a thought....so we've since said that if she can come and do as she pleases with her sisters things, then she'd have to let her sister play with some of her things....
Just wanted to let you know you weren't alone...If ya ever wanna chat mama, email me....