Sounds like you are doing the right thing with the lawyer letter. Sounds to me like she thinks this behavior will get a reaction, like her daughter's to run to her with I'm sorry. It just sounds so much like my own mom's behavior -- to lash out for attention.
I hope your girls see it for what it is, keep talking them through it. I hope your W learns to stop it.
Me 48 H 46 S 11 M 2004 BD 8/13 H moved out 2/15 -live in the present, enjoy the beauty around and within you, explore your new future-