Thats the thing. Call your attorney tomorrow and ask about case law and no third party interference where you live. Tell him/her what you hear here and ask, why not?
The evil me, but honestly I almost wonder what WOULD happen if dad could/would choose to bring SM in as Mom??!!! That would clarify a few things quickly! I wish, I wish.
My take, as a humble member, is once you are going to court, ask for what you want (much better to do in writing in advance). State the provocativeness of SM's relationship to child...be honest and straight and just ask what you can do? (forgot, you got an attorney...so what is up for how invested the attorney is in this issue and how he is able/willing to speak up about it).
One solution could be as simple as requesting a clause in the court order, that neither parents or any stepparent make ANY derogatory or inflammatory remarks ABOUT THE OTHER CHILDS PARENT to the child (state names of children). Period. Not saying this would end the story, but its now in the court order.
I agree with the idea of therapy for the kids. The sad thing is this is confusing, and crazymaking behavior for kids (amongst prob another 20 emotions) and seeing someone weekly can help. The other sad thing is the older the kids get, the more you give neutral responses to their fears, the more their fears will lesson (good, mom not freaked out, I don't have to be) and they will learn on their own to get dads tune. Thats the sad part, but part of growing up.
I LOVED Isabella Soriano's response to her children, " I think your dad may have misunderstood what we agreed on". I have no doubt, the understatement of the week, and no drama for kids, plus a message of, "I'm cool with this", I am in control... I am going to write that one down!