I experienced that with 2 of the steps....There are no tips, stay out of the way. They grow up into awful teens and then criminal adults

The father and mother will parent as they see fit and you have no say.

What i did is that i removed myself completely of the equation. Got a second job and made up activities or obligations that I had specifically during their visits. Of course in the beginning, my husband was a bit mad and accused of me of refusing to be a family but I stood strong and continued on. He eventually understood that the weekends for him are on his own and I wont be there to take care of anyone. If I happen to be there, i do the bare minimum.

The youngest of the steps was 4yo when I got married and I did it all for them in the beginning but they were so poorly behaved and my husband defended that and gave in every time that the only way I could make it was to disengage completely. None of them miss my involvement and they are delighted at my absence

It does get old to have to go places and make up activities and have a second job on weekends when you should be rested but this is the life I chose when I married someone with kids 

My tip is that you refuse to watch SD if she does not listen to you and your SO doesn't back you up. No responsibility without authority.  You're NOT a mom of two. You're a mom of one and an unpaid nanny of one.  Do you have a paying job or are you financially dependent on your SO?

If she gets lippy, she gets a march to an isolated corner where she can stand and hold the intersecting walls together with her nose until YOU get tired.

As for your idiot husband. Do you really want him polluting your own child with his shit parenting?  

Tell failed daddy that either he steps up and parents before you have to or... you wil make it happen and neither he nor is ill behaved failed family spawn will like it.  Either way, there will no longer be a void in discipline and parenting in YOUR marital home.

He needs to put a hand between his legs, grab a big hand full of man sack, and  man up.  He sucks as a man, a mate, and a father... by all indicators.

smh

If ithe failed family spawn is in YOUR home, he is there. Or the spawn is not. His choice.  No more Skid invasion unless daddy is there to take care of her and the results of his shit parenting.

IMHO of course.

"She doesn't listen to me because when I say something her dad will overlap me and let her do whatever."

Nope. If he is home, he has to do all the childcare for her, then. If he isn't and you are home with your baby, he either backs you up or finds childcare for her. 

Obviously this is a difficult situation.. you barely have been in your relationship long enough to really know your partner.. and you already have had a child with him.. just THAT would be stressful enough.

But you add a 4 year old.. who by the very nature of a kid that age can be difficult.. bratty.. not able to express emotions and frustration in a mature way. I mean.. what on earth would she be seeing.. and expected to accept.. a new parental figure.. then a new baby.. her and probably fairly freshly after her parents split.

And.. her dad basically hands her over to this new person who is already overwhelmed dealing with being a new mother.. and doesn't have the bandwidth or patience to deal with another child when she has barely got her feet wet with her new baby.

I get you are at home now due to your baby being young.. but what would your SO be doing if you were not in the picture?  who would be caring for his child during the day.. who did it before YOU were there?  

Maybe his custody would need to be adjusted so that his daughter is there when HE is there?  weekends etc?  

Because it sounds like you are overwhelmed.. and his disney dad actions are not helping.  What does he say when you tell him it's frustrating to be undermined when he comes home and unwinding your decisions?

If he trusts you to watch his child.. (you do not have two children.. HE does).. then he has to trust your judgement on how you set boundaries with her.. and consequences to a reasonable extent (NO hands on punishment... she can sit in corner.. in her room etc.. be made to help clean a mess she makes.. but no swatting.. it's too risky to be accused of abuse).

Tell SO:

"I cannot have full responsibility without being a team and you backing me up. I cannot have full responsibility without full authority." Period. Either he gets on the team, and backs you up or you disengage and he does it all, while you focus on your bios.

Kiddo cries and doesnt want food youve presented? She goes hungry.

Daddy cakes wants to give her everything after shes gone full tantrum mode? He gets to do it all then. And you keep your finances separate, of course, and YOU do not support kiddo.

This sounds very stressful and it doesnt get better over time, it gets worse.

You dont like the kid, simply because shes unlikable. She sounds horrible.

If SD won't listen to you, and DH has tied your hands on disciplining her - then SD needs to be at BM's, with DH or he can pay to put her in childcare out side the home. You must be exhausted. You don't deserve to have to deal with a newborn AND his little snot-princess. 

Although you are tired, pull your strength together for a minute and face DH. Tell him what you want to happen, and don't give him an option to weasel out of it. The only consideration DH gets is that he can take "24 hours to figure out the logistics, and let you know."  That's it. That's how you deal with a snotty skid and a pussy DH. TAKE,.....NO....... CRAP!    

I hope you get some much needed sleep. Good luck with your DH. These men need to be reminded that they have consequences for their kids' actions - and its their responsiblity, not the stepmoms.

Stand your ground with SO now! Tell him you will no longer be looking after SD and that will remain the case until he steps up with the parenting to sort his daughter out. 

This is a four-year-old. There has to be some grace given. 

My simple rule when I got married was in line with my wife and that is any child that comes into our home whether it be SKids, friends, cousins, or bios has to submit to our parenting/rules. 

Talk to your SO and let him understand your expectations. If his daughter comes into your home then you expect a certain behavior. If she doesn't listen then she is disciplined when he is not there. Have him decide the punishment he wants you to enforce. A four-year-old needs consistent discipline but not excessive. 

If he does not want you to carry out his discipline, then she does not come over unless he is present. Obviously do not swat as someone mentioned earlier. 

This is just common sense. This is one of the first things I wanted to know about my wife. What level of discipline could I employ? I was a school teacher and basketball coach so I had no issues disciplining, but I wanted to be in agreement. If she had said no discipline, then I would be married to someone else right now.

Who had a young daughter,  the same age as my own DD many years ago.  We even had similar schedules,  so I thought it was great....until I saw how manipulative this kid was. If her dad was out and I asked her to so something,  like pick up toys or something along those lines,  she'd say 'I'm gonna tell my dad"....once I bought myself,  my daughter and her new hairbrush.  This child actually accused me of switching mine with hers.  They were the same, so I don't know where she was going other than just making a problem,  so I I explained they were the same and offered to put her name on hers....again with the 'I'm gonna tell my dad".... I got my phone and said "yeah, I think we should call him"....when he picked up I let him know she wanted to talk to him. She froze and started crying.

That relationship didn't make it and it was partly because he was a Disney Dad that lost his mind when the princess frowned. 

If you are providing care/supervision for a child, you have to have authority to act on discipline.  Behavior standards are not novel ideas and he will have to get on board or figure out other arrangements.