Oh my goodness! I read what happened when you first posted it and I just read it again. I'm so sorry for the things she said to you. I'm guessing that there's no just sitting down and talking to her. I can totally understand why you wouldn't want your husband talking to her! However, he's a mamma's boy. He's not going to just cut ties with her.
As I see it, here are your options..1) have a sit down with your husband SIL and MIL and try to come to a grown up solution 2) give your husband an ultimatum (a horrible idea, btw) 3) just let it go. If #1 isn't an option, then you have to go with #3. You have to. If she hates you, why do you want her around your kids anyway? My stepmom and I don't get along. We tried to fake it for awhile, but we just don't like each other. There's too much resentment from the past. She was not very nice to me when I was little and has never apologized or admitted that she was wrong. If I even look at her funny she won't talk to me for months. So, I just let it go. I don't want my kids around someone that treats me like that. As they grow up, they're going to realize what's going on. At that point, I'll try to explain.
You're wasting a lot of time and energy being mad at them. You are the only one that can control how you let others affect you. I definitely think you should talk to your husband. I would want to know if she's using these visits with him as a bash fest on you, or if it's just business as usual. If she's bashing you, is he standing up for you?
I guess there is an option #4. You could suck it up and apologize to her. I don't really know what you did wrong, but maybe an "I'm sorry for offending you." would smooth things over? I honestly don't think you were in the wrong, but she probably doesn't see it that way. If she just wants to be right, then that would probably make things better. You'll just have to grin and bare it, I guess.