How is your DH with her?  Does he share your assessment?  Can you talk with him about her issues?  I'm guessing he is like so many dads on here, blind to their child's behavior.  Or, like my DH, putting the best possible spin on it.

Take your baby and go. No child needs to be exposed that that excercise in failed prior family breeding and parenting.

So bio mum being hardly present in her life is no excuse for the meltdowns and tantrums. I've seen it with my own cousins kids, they've been allowed to rule the roost since they were toddlers getting their way, the tantrums etc is their manipulative tactic to get what they want whenever they want it because daddy has allowed and encouraged this behaviour. 
 

i've been raising my 4 & almost 6 yr old alone for the past 2 yrs as been forcibly separated from my husband working overseas in essential services due to my countries border closures, i do not tolerate this behaviour and my kids know tantrums and meltdowns don't work. Being effectively a single parent raising this kid majority of the time is no excuse.
 

Doesn't sound like your BF is parenting her effectively.  Or if she has developmental issues, has she been evaluated? She's only 5, none of this is her fault. 

So BM "isn't that great of a person" but how is she as a Mom? If the child is safe and loved while in her care and a "good enough" Mom then this child should go to BM despite BM's other issues.  No need to live in misery with your own BK. I doubt your husband loves dealing with screaming banshee child either.  It is hard to say if your relationship is worth salvaging until you can get enough space to see how your DH is without the screaming kid around. I am very happy for my DH to pay extra support to have the SD's out of my house. Everyone is happier. BM and SD's are birds of a feather which is why they fight constantly with others and each other. 

I know its a rant rather than asking for solutions, but basically it sounds like your BF is not parenting his difficult child. If BM is ba enough that she lost custody of such a young child, he has to step up and start disciplining his child. Its not her fault shes not being parented properly. So many times we direct our feelings to the child, because its somehow "safer". When they are older they do cause misery and they are responsible. I dont think a 5 year old should be allowed to make everything about her, or be allowed to manipulate her father. All that needs to be shut down immediately.

Sorry if this sounds harsh. Also, if you have no luck in bringing your BF on board with parenting, time to perhaps think about alternative plans. These behaviors wont get better.

In kids that age, behavior is usually "trained" into them. If DH has 90% custody and BM is mostly uninvolved, i'm guessing SD's childhood has been chaotic. Someone has inadvertently "trained" SD to act this way.

You said you have been a nanny, idk how much experience or training you had or with what ages, but i can tell you, as a SM, there are forces at work that prevent you from using your previous knowledge and skills to help SD even as much as you would be able to help a child you were nannying for. I had a lot of previous experience with kids, too, but at this point i've thrown my hands up with most of my SO's kids. Between BMs planting seeds of distrust and dislike, guilty daddy who would rather "have" the kids than do what's best for them, and the microscope of people looking at a SM's actions and hyper-judging, it's so much harder.

Your DH is the ringmaster of this circus. He has to want her to behave better and be willing to first learn what it takes to make her better, and then be willing to actually do it. I would suggest having an honest talk with him about her behavior. Don't frame it as you hate her. Frame it as you have experience with kids and her behavior is more problematic than any you've seen before. It will likely cause her negative consequences in school, at home, and in future friendships and relationships unless she can learn self control and socially acceptable behavior. This is all true, btw.

If he is receptive, maybe you guys could do therapy and parenting classes and also get the child into therapy. You both have to be willing to follow through at home, too. He can't cave and undermine when she gives him the sad eyes. That's the only way i see this ending with you guys staying together. It may help you parent your own little one together, too. 

Your dh is responsible for this mess. If BM is barely involved then his poor parenting is to blame here, not hers. I know it is easier to blame stepdaughter and biomom but they are not to blame here. Sorry! That's hard

WTF does he think therapy is? Some kind of torture? Punishment? What century is he living in? 

If he loved his daughter, he wouldn't want her to suffer with her demons, he'd want to get her the help she needs. Tell him to wake up and stop making everything about him because that's what is bothering him. He's scared that people will think it's his fault that his daughter has'problems' when it's quite the opposite. People are going to wonder why he is twiddling his thumbs and letting the child suffer the

I don't understand why he thinks therapy would be traumatic for her.The way she is living NOW is traumatic, and it needs to be fixed so that she has a decent chance at a happy, fulfilling, productive life. Your DH is setting her up for absolute failure if he doesn't help her cope now. I would think the first step would be to have her evaluated and let the professionals guide you on what kind of help SD needs.

You've said several times that your DH is a "good dad." But his behavior and unwillingness to address the issues with his daughter shows that is is NOT a good dad. He might love his daughter, but that doesn't make him a good dad. And by not addressing it, the whole household is turned upside down, making it an unhealthy environment for his other child. His inaction is actually harming BOTH children.

But you can't force him to do anything if he's got his mind made up and he's unwilling to hear other opinions.  But you CAN set boundaries around this for yourself and your child, and working with a therapist would probably be beneficial.

No wonder you are angry. But direct that anger where it belongs: with your DH. The child's behavior is a symptom of poor parenting (although she might have other issues as well, hence the need for a complete evaluation).

Man, this sounds so familiar; I lived this situation for many years.  DH was the same way with SD.  He had custody, she was manipulative and destructive.  BM wasn't in the picture much and we had kids of our own.  I had 12 long years of disrespect from her and DH who insisted I knew nothing of parenting. Then when she became a teen, she was promiscuous and having abortions when she got pregnant. I emotionally and psychologically removed myself from the situation and concentrated on our 2 children.  When SD finally got so out of control and became a danger to my kids,  I had to remove both DH and SD from the house ( I owned it before we married so I could file a forcible detainer to remove both of them).  he ran back to his parents with SD and lasted about 1 or 2 days alone with her. He couldn't handle her.  He ended up leaving her with his parents and moved back after a very long discussion that he could come back but she couldn't. I refused to let her back. We came very close to divorcing over her but we have actually celebrated our silver anniversary recently. 

As so many have said, this is poor parenting on his part and could actually be considered neglect. As long as he parents this way, this situation will not improve.  For me, I had to make the decision of my children or him and SD.  My kids won out. And eventually my marriage won out.  I hope it doesn't become that way for you but you may have to decide how much you can tolerate and draw that line in the sand.  I would suggest you record her behaviors and save them some place safe in case you have to take the baby and leave.  You can use those recordings as proof that your child visiting there would be a danger to the baby and see if you could push for his visitations to be supervised. 

So sorry to read what's happening. I can totally sympathise with you. I wish i could say it will get better but i dont think it does. My SD was just 4 when i met her and shes with us half the week. Her Dad moved into my house with me and my son who was 6 at the time. I found it difficult to adjust to living with him and having her around so much. I didn't really like her and found her to be a spoilt attention seeking brat. I read about the trials and tribulations of being a step parent and hoped that it would get better. 3 years down the line and nothings changed. I dont like her, i can occasionally tolerate her, not always, i feel like shes ruined my home life and it scares the hell out of me that this is it for the next 11 years and beyond no doubt. All i can say is that excepting that i dont like a child has been a difficult journey because it goes against everything I've been told and believe about children. I'm supposed to be her guardian. But i just dont like her. I didn't like her 3 i dont like her at 7 and I'll probably like her even less as she gets older.