Birth mother vs Step mom

Ashlea • 27 years old... Married the love of my life 5.20.15 and gained 3 amazing step kids, baby #1 due May 2017
So I have 3 step kids whom I adore more than life itself and my husband and I are in the middle of a custody battle of epic proportions with his ex...
A little background information, he met his ex his junior year and she was already out of school. Anyways she was controlling individual who got pregnant and used his daughter to make him stay, she then got pregnant 3 more times, one of which she decided not to have because it wasn't my husbands and he said he wasn't going to raise someone else's kids. He left her a few times but came back when she'd start keeping his kids from him. I might also mention that his some has a heart condition and she gets social security for him. But this all brings us to the custody battle, he left and we started dating at first things were ok but then I met his kids and she flipped out (her boyfriend of a month was sharing her bed with her and my step son, who's 3 but she doesn't allow to sleep alone and refuses to let him have anything but a bottle and refuses to potty train or send to preschool) because we had been together that long (almost 4 months). But as we got more serious she got vindictive, stopped telling him about doctors appointments and then stopped allowing the kids to see him once we got engaged and decided she wanted to move them out of state to live with boyfriend of 2 months. That is when my husband got a lawyer. 
We've been married for 5 months now and she started acting nicer to my husband once she was served papers but she still treats the kids poorly, she smokes pot with them in the next room, she makes the 10 year old take care of the younger 2, ignores the 8 year old and makes the 4 of them (her and the 3 kids) all bath together. She is jumping from guy to guy, lost her job, refuses to get another because she thinks my husband will have to give her more child support, plus she is getting unemployment, the social security for her son, and watching children in her home for cash, plus whining and crying to all my husbands family about how she can't afford clothing, food and the house and getting them to give her money as well. 
She tells anyone who will listen that I am trying to take over her life and steal her kids, which was never my intention, all I wanted to do was get along in the beginning until the kids and other people started telling me about what she does. Let me clarify that I still don't want to take away her role as their mother, I just want them in a safe and secure home environment where they can be kids and not have to worry if mommy is going to scream at them and send them to their room for no reason or if they are going to have clean clothing to wear (they never have on underwear when they come to stay with us every other weekend as per the original first court hearing) or if they will have dinner (though my husbands aunt is apparently now supplying all the groceries so that's not an issue anymore) and I hate hearing them talk about how all mommy does is sit on the couch on her phone drinking and doesn't do anything with them and I hate seeing them get upset that they have to go back. Those things really get to me and break my heart but it was never my intention coming in to the kids life to replace their mother.
Anyways she's at her lies again trying to say that I'm plotting against her and that my role as a stepmother doesn't count in the eyes of the law until my husband and I have been married for 2 years so I am not allowed to be around the kids without supervision. It's so much stress, every day it's something new and I feel like it's putting a negative affect on trying to conceive. I'm really just trying to be a good step mother, I went back and forth about having a baby of my own, which has been something I've always wanted, because I didn't want the kids to feel unloved or replaced or anything else they will be told if I get pregnant. They were my first concern when buying the house that we now own, making sure they finally had space of their own. They are my first concern in any major decision I make and it just hurts so much that I am being made out to be some evil vindictive monster trying to ruin lives to anyone that will listen. I just wish the lies would stop.
Am I wrong for wanting what is in the best interest of the kids? What would you do?