I think you'll have a better chance if you take a backseat approach. You are the new wife, but it doesn't make you a mother to them (not suggesting you think you are). But sometimes step-parents can get confused and think that they deserve to be in the parental role, and if they want to ruin things, then that's a great way to do it.
I would suggest approaching your relationship with them very carefully. Leave disciplining to your husband, unless it has to do with you or your child. As in, if they are doing something to your child you don't like, you definitely should speak up since you would to anyone else.
But, be prepared for a lot of emotions from them. I don't know the situation of their mother, but remarriages are difficult and they might want to take it out on you. I would suggest being patient with that...no matter how unfair or difficult it is to you. If you really want a relationship with them, you'll NEED to be patient and give it time in order for your relationship to grow. And, you'll need to approach them as a friend more than a parent (not suggesting you act like you are 12-13 yrs old!)) Maybe more approach it like you're their aunt.
My dad left when I was an older teen. My mom remarried about a year later. She had it in her mind that since he was her new husband that somehow he was our new dad. It doesn't work that way. Sure, our dad was gone for good, but he was still our dad. Step-dad approached things almost as wrong as he could. Everything revolved around him. My mom went with it for the sake of the marriage. He ended up kicking me out, my younger sister moved out right as she turned 18. And my next sister was kicked out at 17. The ironic thing is he was supposedly a child counselor, but yet, he never practiced anything he claimed to know. To sum it up well, he was an "evil step-father" in our view, and he ruined so much of our family.
I guarantee that if he had taken it slow, not tried to force the father thing, and not blamed everything on us (when most of it was truly his fault), things would have gone better. He wanted to be called Dad, and if I didn't, he would act all hurt and dejected. I didn't know this man! He wasn't my dad. So, I didn't call him anything. If I had to talk to him, I would walk over to him and start speaking, calling him nothing specific. So, don't ever do that;-)
I'm not personally a fan of remarriages. I've only shared the tip of the ice burg here, but what we went through was awful and nightmarish. It took me a LONG time to work through the harm done then and move along. Due to what I've gone through, I can almost guarantee I will never get remarried should my hubby die or if we got divorced. I have kids now and they are my priority. I never want to chance them going through anything like I did...then again, I wouldn't make the same mistakes my mom did since I have experienced it from a child' point-of-view and she never did.
I'm not suggesting you shouldn't have married him or that everyone should choose what I'm choosing...just suggesting that remarriages are very difficult.
Your husband is right to keep some things just with him and them. That is very important. I haven't read other responses yet, but DO NOT INTERFERE in everything. They NEED to have time ONLY with their dad. You are his wife, not their mom, and they need time with only him. My mom stopped doing that. In fact, she backed off so much that we felt like we had no mom. She was almost a puppet for the man she married. He had say in everything and didn't have the natural love for us that a parent has, so it was pretty awful at times. I can't tell you how badly I wished my mom would have been there for us and made us a priority to her. After my dad left, she did an AMAZING job. Then she got remarried...and we suddenly meant very little to her.
So, anyway, approach it slowly. Be understanding of their attitudes. I would suggest ignoring them and showing them love no matter what. Sure, some might say that it's allowing their misbehavior, but I can guarantee that if you get on them about it and fight back, you're only setting yourself up for a complete downfall and no growth of relationship. Just start slow and ask each girl about themselves and about things they like. Show them love by being interested in THEM. My step-dad was only interested in himself and seemed to care less about us.
Okay, enough from me. I probably wrote way too much. I've been having a resurge of emotions about it all because he recently divorced my mom because he was having an affair with a girl that had finally turned 18 (when she was 17 he was involved too!). She's now living with him and they want to get married. He's 58 years old, and she's 18. I think that's gross. I've lost any respect for him that I developed over the years (especially because he has really mistreated my mom!).
Good luck to you...