My new 10yo step daughter is manipulative, conniving, dishonest, fake and a bit of a gossip-trash talker in the family. She falsly victimizes herself and lies to turn family members against eachother. The hardest part is her father is completely manipulated by her.

He tries to coparent w me, but I feel like part of him doesn't know this side of his daughter bc she's so good about being a completely different, innocent victimized sweetheart around him. The other reason is, I feel he thinks if he admits his daughter isn't perfect, then it's the same as admitting he's a bad parent, which he isn't completely awful but he honestly knows just about nothing about parenting. It seems to be pride based instead of being based on her best interests.

She's so smart about getting under my skin while hiding it from her father, youd think she was an adult. She'll give nasty looks and sass right around corner or right behind his back. She lies on everyone in the family to make them look bad, to change the narrative so she appears the best or a victim, even her 4yo brother.

If you ever give her chores or scold her she calls all her family members to lie to get sympathy. She copies ppl around her as if she doesn't have her own identity or to fit in, but it comes across like she's trying to get under your skin or steal attention.  Everything has to be all about her, just like her father. If I have a headache she has a bigger one. If I get stung by a bee, she got stung by 3. If I have heartburn she has heartburn. If one of the kids wants a specific hair cut, she'll go with her grandmother and get that hair cut then tell my daughter no one can copy her hair.

She likes ppl to believe she's not safe w the adults in her life. If her dad finally gives her a tone, he's now unsafe and too hard on her. If I give her a chore same thing. She plays her father and family members against me and tries to play my own kids against me w outlandish lies.

If she sees a kid get reprimanded for something she set up or did herself she smirks. If she sees her father question me or interrogate me over something she said or something I said she did, she looks happy and excited. She has a stance and stares me down and my children (14 and 12yo) until we get uncomfortable, it feels like a death or murder stares.

She manipulated a story to her grandmother to get the word to us about her diaries, so I read them and it was severe cursing and talk of her doing anything to make sure her dad left me. To make it even more scary she's crushing on my son and so jealous and angry the feelings aren't reciprocated she on sabotage mode. She claimes he steals, but after going through her room she's been stealing from all of us and he refuses to look for himself. Another thing she wrote was that she's so mad she wanted to murder one of us. Her dad refused to read the diaries or look in her room. Later I realized that she was writing in several diaries and leaving them open and out in my kids room for my kids to read how much she hated them.

She refuses to bathe or change her clothes sometimes. She tells me no if I ask her to shower. If I ask her to clean her room she starts throwing things until her dad comes around,, then its sweet obedient daughter.

If she's remotely scolded she says she'll kill herself then smiles when her father is upset. She fake cries and her father doesn't even notice theirs no tears. If I give her chores she calls herself Cinderella or fakes illness and body pains later claiming she's slaved.

She takes my daughters handmade jewelry and art and cuts it up. I found her father's art from his youth destroyed in her room and her dad never said a word. 9 times outta 10 times she acts out, absolutely nothing is said unless I say it. If he does say something to her, he doesn't even use an authoritative tone. She manipulates him and negotiates out of obligations, or groundings until she gets her way and has her father apologizing and blaming himself for her behavior.

Now let me give you some perspective. The child's parents divorce 2 yrs ago, I came around the kids 6 months ago right before the mother died of organ failure as an alcoholic. She had been in her second chemically induced coma. Stories from everyone is her mother claimed the daughter and husband was so evil and hard to deal w that it drove the mother to drinking.  This 10yo has not once shown remorse or cried over her mothers death, NOT ONCE. The father, my partner owns a landscaping business and was never the primary parent and you can tell by how little he knows about raising kids. I have taken parenting courses, classes and taught parenting classes. I have spent 14yrs studying parenting and continuing to research the best ways of handling challenges w kids. Her brother is 4 and when I came into his life he was so out of control and developmentally delayed that he's now in special needs classes and being tested for autism. 4–6 mos I worked w him on weekends and now he's a completely different child. However, if I share w my partner parenting advice or tactics or ask if he'll read up on parenting he responds with attitude and gets upset talking to me in a condescending or patronizing tone. He has a hard time talking about tough things wo responding aggressively. It's all very wild to me. As far as his daughter goes, no one is remotely rude to this child. No one even raises their voice to her. She does a fraction of the chores and obligations as the other kids and is spoiled more. Everything is based around what she wants. Dinner, weekend plans, chores and entertainment. If she doesn't like burgers, wdoctor her wings. 6 months ago was the first time the child had been grounded or reprimand for anything In her life. Her father has such a hard time believing me that I'm scared to put my foot down out of fear he'll argue with me for hours or days at a time.. Im scared she'll come up w a lie that puts me in jail or has me and my children homeless. She's the only thing destroying my relationship and my home. Me and my kids are scared to be alone w her bc theres no telling what she'll lie about and how her father will react bc he'll believe anything she says even when she's caught red handed.

Before we met she called the police on her father and made false and exaggerated accusations that had him removed from the house and had to plea down the charge. All later proven she lied. Just before her mother was admitted into the hospital and died, she was facing charges and lost custody over lies this little girl made up. Like her mom being an alcoholic wasn't bad enough, this little girl has to make up insane lies.

I feel like the root of the problem is my partner not communicating with me in a healthy way and not being able to coparent productively staying on the same page as me.His disfunction is literally training her to be this bad, when he's not giving her bad examples hes enabling her behavior. 

What do I do to help this little girl before she gets to puberty and is completely out of control and help my partner parent w me before she breaks up our family?