My BF and I are currently in custody battle with BM.
BF = Boyfriend, that means you are not a Stepmother yet, even when you are someday you have no legal rights to this child.
I think I just found my own answer:
I'm in a similar situation.
When BM has a bad reaction to something me and my SO do that has nothing to do with DSD (and it's rare for her to have a bad reaction for something that IS to do with DSD)
I find ignoring her to be the most effective. She has a rant, she's let off steam. There we go, it's all been said and done. No need to drag it out.
Ask me anything :)