I think we need more specifics - but for a general answer, there isn't much you can do about her behavior. The only person you can control is yourself. The only suggestions I can come up with based on your post is don't involve yourself with BM, let your DH handle her...disengage, disengage, disengage.

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“Some cause happiness wherever they go; others whenever they go.”

I also tried to be civil, even friendly at first. I went so far as to organize a dinner with BF, SD, BM and BM's boyfriend to celebrate when SD9 got her first stripe in karate. All I've ever received in return is grief given to BF about absolutely everything, even down to the type of shampoo we use. I have no dealings with BM, only see/speak to her when I absolutely have to and have even gotten to the point that I don't want to know about texts or emails from her anymore. She makes everything as difficult as possible and I can't allow her to "rent free space in my head" anymore. We always talk about disengaging from the Skids but there's definitely something to be said for disengaging from the BM too.

"There are two things over which you have complete dominion, authority, and control over - your mind and your mouth".