“The ONLY issue we have is the youngster.”  I’m afraid not.  A youngster is a youngster and is being molded by the adults around her, particularly mom and dad.  My read is that neither you nor the youngster, per se, are really that guilty of anything.  I’m saying this as a bioless SM, like yourself.

First of all, from what you stated above, it appears that BM is more than likely poisoning the well against you and probably her ex-husband too.  This line, for example, “You're just my babysitter; I don't have to listen to you,” reeks of something a BM might say to her child, purposeful, in an attempt to make her love her more and her father (and esp. father’s GF) less.  Look up Parenteral Alienation Syndrome.  This can be anything from BM rolling her eyes whenever bio-dad’s name is mentioned, to BM making up out-and-out crimes to keep her child from dad.  It can happen with dads going after moms too, of course. 

You made this comment too, “She (SD) punched me as hard as she could, square in the kidney.”  Followed by: “That's how she was raised to handle people she don’t like.”  So, as evil as some of this may sound from SD, she is more than likely just picking things up from BM (and bio-dad, I’ll get to that) and rolling with it.  It does not excuse her behavior, however.

What is your DH/ BF doing about this?  It sounds like at least initially he was the typical patsy.  You mentioned, “She (SD) argued with me that she can do what she wants too and her dad said so.”  You also stated, “Every day after that, when he left my name became "maid". I told SO about it and he said it's just a kid thing; it would pass.”  Later, you mentioned in the midst of some sort of drama, “She starts telling me that she wants me to die. She wishes I would just kill myself or that someone would kill me.”

So, while all of this was going on the last couple of years, what did your DH/ BF do about it?  It sounds like he pretty much gave into her and put up with it and expected you to do the same.  This let her know for a good couple of years that you were her competition.  It let her know that she ruled the household and not you.  It let her know that her BM was right—that you were just her babysitter or maid—and that she didn’t have to listen to you.  Her father unknownly? backed her up, apparently, on all of this.

Now, it appears your SO “may have” and that is a big “may have” finally gotten the cajones to start to deal with some of these issues.  You mentioned, “SO is at a loss too. He does what he can to correct and mediate behavior but nothing works.”  What has he done?  He was a patsy, lackadaisical parent and now he thinks a snap, harsh comment or two aimed at her is going to correct her behavior?  Has he sat down and talked to her calmly and explained things to her?  Told her he loves her?  Over and over ‘til she gets it?

I’m sure this probably isn’t what you want to hear, but it is very important you realize that your SD herself is not the only issue.  Manipulative, controlling BM and weaker, enabling DH = step hell.  DH cannot go from one extreme to the other with his child, go from patsy to whip-snapper and not expect his child to be confused.  She sounds very confused to me.  HOWEVER, it is not your job to correct the mistakes mom and dad have made.  They have to own that.  You were correct to stop babysitting their child, since apparently neither one of them had the ability to teach their young daughter respect for herself or for others.

You are dad’s GF or SO and your SD is dad’s child.  These are two different kinds of roles, two different kinds of love.  No sort of competition should be going on between you two.  But, unfortunately, both mom and dad set the both of you up for it.  Dad will have to really man up to correct this one, because it sounds like BM is a nutcase.  Just remember: Manipulative, controlling BM and weaker, enabling DH = step hell.  And, it is rarely, if ever, a child’s fault when they hate SM.  I’ll put the burden on any SK’s shoulder when s/he is an adult.  However, when a child hates SM, it is usually because both BM and bio-dad have either accidentally, on purpose, or accidentally on purpose set it up that way.  And in your case, they have.  BM was/ is into parenteral alienation and bio-dad just sat there and let(s) his daughter act out with little to no repercussions.   Family counseling. 

Yea, love would not be enough to keep me aound that psychopath. 

If I was you, I would move on. That level of chaos is a no go for me. Have you thought of livimg separate and just dating? 

Sorry you are having to deal with such a difficult stepchild. The first mistake you made was agreeing to watch the child on your own. That's called Responsibility without Authority. Change that first and hopefully things will get better. The next thing you need to do is to have very clear boundaries with the girl. Explain very clearly to her how you expect to be spoken to/treated. The moment she crosses those boundaries address it with her right away, and don't wait for the father to do it. The third thing is, do not let her dictate how you will use your time, emotions, ane energy. You focus on yourself, and if there is left over time and energy, maybe, just maybe you'll spend a little on her. Do not just go along with what she wants. State very clearly, that's nice you'd like to do that but I prefer to....or that's nice you like that color, but I'm going to pic this color. She is not the center of your life.  Then, ignore her stupid lies that she tells the family about you. Go about your business as though you never heard them. She needs to know she has no impact on your life. And, never ever trust her enough to have a genuine conversation with her. Do not let your guard down. She can never be trusted. Show her who's queen.

when she goes psycho and calls you the maid/servant and whatever psychotic meltdown you send the video to her dad and say “handle this now!!”

It sounds like parental alienation from BM but your real problem is your SO. What is he doing to correct her? protect you?  Teaching her to respect adults? What behavioral services did he contact? She's threatening you - were the police contacted? 

Your SO is not protecting you - get out now. 

Now, fast forward about another 2 years and here we are.

Good Lord, why? Why are you still there?

Does your DH have a visitation CO. If not, he needs to get his ass to court pronto.  Does your DH have actual balls or are they in the little girls back pack?  Apparently they can be found in the back pack.

As for the demon spawn.  You are the adult. You have lived the ages she has and know what goes through a kids mind. So go truly evil SP on her toxic little ass.

When she starts in on her hopes that you die crap tell her that it is too bad that abortion cannot be applied retroactively to a toxic sex trophy after it is born or she would have been put down the garbage disposal in pieces when she first opened her toxic vitriol spewing food hopper. (Just venting of course.... mostly at least)

No more of this tolerance crap for this little shit.  Make her life a living hell. Show her how you deal with people you don't like.

Her life should immediately be of two formats. Moderate misery for decent behavior and abject misery for her toxic crap.  Introduce her to the twisted ear mach to the corner where her nose is planted with a firm swat to the ass where she will stay until you get tired.  If she makes a mess, grab the paddle shaped cutting board and follow her around with it directing her clean up and applying the board to her rump any time she opens her vitriolic lips.

Teach her what the life of the spawn of a shallow and polluted gene pool will be.  And .... web cams.  Keep her ass under the electronic hairy eye ball to review with daddy when he gets home. Every vitriolic word.  Put the cams on your phone to shut them down when discipline is required.  No need for that to  be filmed. Corporal punishment is legal in 49 states but there is no need to cause yourself any problems.

Own her toxic ass for every second from the minute she sets foot in your home until she leaves to return to BM's.  

If I or my brother had emitted those words to our parents we would have been fertilizer. And it would have only happened once.

I am fortunate.  My DW is a quality partner and a quality parent.  She would have shredded her kid if he had pulled this kind of crap with her... or with me.  So I would not have had to.  And... my Skid was comparably an angel.  

Your DH is a POS and not worthy of you , boot his ass and put he and the progeny of that toxic genetic coupling to the curb.  This is all his fault. He is a failure as a parent. He should be the one whupping this toxic kid's ass when she pulls her toxic crap and he should be the one making sure her nose is semi-permanently planted in an isolated corner. 

This kid should tremble any time she even thinks about misbehaving.

Oh hell no.  Not in my world. No more of this every weekend crap.  No more than EOWE and then only if she earns it.  This kid has not earned unconditional love and support. Everything about her life should be conditional to her behavior.

This kid has me grinding my teeth and all I did was read your post.

Beyond belief.

smh

It is common for children who feel out of control because of environmental factors. Ex. Mom in and out, not stable.  They don't know how to to feel or act because they dont want to not be loved by the inconsistent bioparent. To act out in ways to gain control. I highly recommend reading Common. Sense Parenting, it is based on the same model used with children in psychiatric settings.  It is very effective and helps keep emotional responses at bay. Whether her issue is severe behavior or psychiatric it is still effective. 

If not, I suggest you get one.  Seriously.  This kid can put you in a position where you might be facing jail time.  

I hope you understand that this girl will stop at nothing to damage you.  She's already lied and said you beat her.  She will continue along that narrative until authorities get involved.  It's only a matter of time before she "slips" and makes some kind of comment/lie to a mandated reporter, like a teacher or counselor.

Then you will be facing potential criminal charges.  

I hope this man is worth your freedom and future, because you may have to eventually give it up for him.  And be fully prepared that he will take her side.

The fact your SO slept in his mother's bed until he was a teen is another huge red flag, and is incredible dysfunction.  Beware!