I totally get what you're asking for here, and it's wonderful that you care about your soon-to-be stepdaughters so much!
I have noticed that moms here on mampedia get VERY sensitive when it comes to stepmoms and their issues. Moms who are NOT stepmoms, and NEVER HAVE BEEN stepmoms seem to get upset and angry that a stepmom might want to do something for their stepkids and always advise to step back because they are NOT YOUR KIDS.
I'm a stepmom, and I say that God put you in your stepdaughter's lives for a reason. Sometimes that involves stepping back, and sometimes it calls for stepping forward and pushing for what's right. If your stepdaughters fall in the wrong crowd at a bad school and their lives take a bad turn, well if you knew what the right thing to do was and you didn't do it then you're to blame as well.
It's totally right to allow your husband to do the speaking (and you don't mention doing it yourself, which is good) but he will need YOU to be strong for him and his girls.
I do advocate trying to keep the courts OUT of your personal situation. It's much better to work things out between families than to force the courts to choose one way or another. You don't know that they will side with you and even if they do, it causes resentment. As much as it sucks, it's a better route to show her that both of you are on HER side. Flatter her. Compliment her on what she's doing right. Show her you understand her situation and you'll be surprised at what she'll be willing to compromise on. Support your husband, and lead him down this route with his ex. That's your job. I would avoid trying to "convince" her of anything and just find out what issues she has. Talk to her. What does she need and want? You'll be surprised that what she needs could be solved by doing things your way!
That's what worked for us with my husband and his ex. We not only convinced her to let her daughter live here the majority of the time but we're homeschooling her. We thought she'd never agree to it but using the above tactics she did!
I'm pretty sure you're NOT talking to your stepdaughters about their mother. I could give anyone an earful about my SD's mom but I never talk to her about it. That's another sensitive point.
Just realize that some of the moms on here are NOT stepmoms and are extremely sensitive about the bio kid issue. They would not want a stepmom meddling in THEIR issues with THEIR kids. So try to understand why the flame you here! I just see that you care deeply about your stepdaughters and want to help do the right thing. You have a tough job, and hats off to you!
Good luck!