I guess the best way to get answers or suggestions on what to do would be to explain "what" isn't working.

I mean, his EX... you don't have to have a relationship with her.. so theoretically, that should have fairly little impact on your life.

What is your custody schedule.. what is the problem with the kids.. other than they "exist" that bothers you? 

Does your SO expect you to be mom to everyone? are you a SAHM or do you also work outside the home?

Do you really think your feelings towards his ex is why you don't like the kids? if so, you probably need therapy to help you focus your feelings towards the correct target.  In their own right, they don't deserve to be blamed for having half of her DNA.

Does your husband parent well? or is he a disney dad?  what behaviors in particular need fixing?

You won't find judgment or "you knew what you were getting into" here.  A lot of people seem to ignore red flags going into a marriage and then find themselves stuck.

What is not working in your situation? 

I remember feeling such love for my DH and totally disregarding how I might feel about his 3 SKs.  It never entered my mind how much they would affect my life.  I was so naive.  We had 4 rough years before I went to counseling, a last ditch effort before divorcing.  I remember feeling so ashamed, too.  How could I put my 2 kids thru another upheaval, not to mention the effect on DH and his 3 kids who had been thru a lot with BM.   Counseling might be something to consider.  It changed my life and 6 other lives: DH and the 5 kids.  Good luck.

I would also suggest counseling.  I held such more anger towards BM and she really didn't have any affect on me.  I didn't interact with her unless she started it and was rude and it went all through me.  Some of the things that the skids did or happened I blamed on her.  Now I look to their dad because that's their parent.  I'm not.  It really helped.  I'm considering counseling and I think you should too even if it's just you.  

No roasting whatsoever here. There's no really understanding it unless you're going through it. And if the X is high conflict, which he didn't mention but it's often true on here, it's frankly unfathomable for people outside their relationship to understand.

without knowing more details on why, I totally echo the above about counseling for yourself for your own clarity and also as a couple, which can be so helpful. I'd also say to that you've got a really young kiddo, and the four-year-old, even if she's a terror, it's also really young. Such young kids inevitably strain a relationship, and all of that is super super normal. Kids are just hard. And other peoples kids are harder. Your feelings are totally valid, but yeah a professional to talk to might give you some emotional tools to handle the situation.

((((HUGS))))) no roasting.

Welcome to step talk. 

Perhaps you could tell us more about what isn't working for you, then we could make suggestions.  Btw, it isn't compulsory to like your SKIDs - I can't stand mine and this has been the situation for nearly 20 yrs!

It can be really hard to separate the anger and hate that you have for BM from the stepkids. Especially if her high conflict ways lead to the kids acting out while they are with you. 

What is the visitation schedule? What is fully going on? 

No judgement on your feelings here!

No judgment whatsoever on how you're feeling. You have a right to feel the way you do. I have moments when I absolutely can't stand the voice of my SS or his prescence too. How often do you see your skids?

No judgement here, and I hope you will not judge yourself harshly either. It's definitely worth considering counselling at least for yourself as a mother and a wife, and as a content and fulfilled person. You didn't choose the step kids and vice versa. Not adoring them is not a crime, and  is very common and completely understandable. Just do what you wish for your own child and what you want to manage for your spouse' kids.  Try to remind yourself that they already have 2 parents, you cannot be responsible for them even if you wanted to be. Feeling guilty doesn't mean that you are guilty. Enjoy your baby growing up and nurture your marriage. That's what really counts.