I've been in your shoes! SD is 15 now. When she was 6 y/o BM also filed a restraining order for me on behalf of SD. I just moved in with DH (BF at the time) and she was livid. BM claimed I "mentally" abused SD and said wild things like "I'm prettier than you" is what she filled out on the court docs. BM ended up not even showing up to court because her claims were so wild and it was dropped. 

BM definitely went on to gossip about me to her "friends". She is from a small town in a different state. We don't know ANY of the same people, just have SD in common. When SD was younger, she would tell me all of the terrible things BM would say, but it turns out SD was stirring the pot just as much. 

I stopped caring because I realized I truly don't care what BM or SD think of me. I know I am a good mom (to my own bio) and supportive wife. I don't need their "approval". Also, if BM continues to talk about you... just know that you live rent free in HER head. And that's the biggest win you can have IMO! Indifference is key. 

It's hard, and I remember being in a similar position with NPD BM, and what made it worse was DH was commuting and working long hours, we hardly saw each other and NPD and the SDs would go on 3 foreign holidays a year and all sorts of music festivals, while DH and I lived in a small house and could only afford one holiday in the UK (where we live) per year. NPD and her vile daughter have flung all sorts of insults at me over the years. However, I kept quiet and consoled myself with the thought that hopefully she would get her karma, eventually.  That time has now come, she is divorced and has no money, while DH and I live in a large house in the country and have enough money to lead a comfortable life.  I am in favour of letting the Universe deal with wrongdoers.  It always does, eventually.  

I have struggled with HCBM for more than 12 years, at this point, and for me, the only thing that has made it better is to not engage.  I had a literal panic attack the last time I tried to help DH with an email, probably like 5 years ago now.  Since then, I engage as little as possible.  DH has to manage her crazy--and SS15's crazy--himself. 

For me, I know dang well that HCBM is human garbage.  It's not that I care what she thinks of me at all, but I was (and continue to be!) terrified about what she will do next to interfere with my life.  Will she take DH to court again and cost us thousands more dollars?  Will she send her kid back to us (we're custodial) unfed, dirty, and miserable again?  Will SS give me the psycho stare for days again because his mother scewed with his brain again?  Or perhaps something new!  There's literally no way for me to know, and knowing that I'm 100% out of control is really hard for me.

Kes is right that karma is real and she will be held to account, but it takes soooooo long!!!!

I find that the loudest parents are the deadbeat parents who have nothing to do with their kid (by choice), but trash the full time parent and stepparent who are doing everything for the kids. In their delusional minds, bashing the active parents and stepparents to everyone and anyone who will listen makes them feel/look like less of a loser, which isn't the case. Talk is cheap but action speaks volumes. People who truly know you and DH won't believe her bullshit and anyone who believes her bullshit isn't worth your time. 

Just remember, you and DH are doing everything for these kids when BM is too much of a POS to do any of it. 

If you weren't hurt by those accusations.. and upset over the stress and trouble they bring into your life.. you wouldn't be normal.

I do try to not care about people's opinions about me.. if they are people I don't care about or respect.. but when she actually draws you into things like the po.. that is hard to ignore.

What I would suggest is that your husband do all of the contact with his EX.... and further.. might push him to be the person who deals with his child.. not you.. minimizing your time and interraction.. keeps it more civil at times.

He bred with crazy.. let him deal.

Zero tolerance and immediate overwhelmingly painful response to their crap works.

Lather.... rinse..... repeat.

If the strike is physical, like any bully, overwhelming disfiguring instant violence ends the bully. I learned that lesson in my pre-teens to mid teens.  I was an easy going kid and I hated fighting. It scared the hell out of me. It still does for that matter.  I was a target for bullies for a few years. Until I gained clarity that getting hit hurts so if I am going to hurt, the bully is going to the hospital for reconstructive surgery preferably in the back of an ambulance.  The magic is, that bully and any others who saw or heard about it stayed the hell away from me after that.  One time per school and bullies were not a problem after that.

I adopted the same philosophy with the toxic blended family opposition. I was nice, until they made it clear that it was time to not be nice. Then, we used every legal, financial, and social means available to rip out their throats, destroy them legally, financially, and in their community, and make it clear that if they were reasonable, we would deal with them reasonably. If not, they lived life altering  overwhelming consequences.  Most importantly, what they received was entirely up to them.

If I were you, I would file a full blown defamation lawsuit, make it public immediately, and be as brutal and destructive to her as I could possibly make it.  Take it to the community, her job, her church, etc....  Make her live the fullest consequences of her choices.

Most importantly, have fun doing it.

No quarter.  Her only option is full, complete, and public unconditional surrender.  Wars are not won by negotiation. They are won by destroying the enemy. Make no mistake, life in a blended family where the opposition is toxic is a war.  We had to go to this level with my SS's SpermGrandHag.  So much so that we had LEOs at her home in front of her neighbors, at her work, at her church, at her family reunions, etc... to collect SS when she violated the CO failing to return SS on time.  Her idiot son, the Spermidiot, was overwhelmingly uninvolved and for some reason the SpermGrandHag took it upon herself to manipulate, control, and attempt to PAS our son against my bride.  Even when DW was a single teen mom HS student and then college student she was more than effective in beating SpermGrandHag into submission. The only advantage that the Hag had was that she and SpermGrandPa carried the big financial stick.  That was until DW and I married. At that point the financial advantage shifted in our favor and did nothing but grow exponentially over the years.  The Hag still tried to do her usual shit. So we became more brutal in our dealings with her. We bared her ass completely. Showing the courts, her job, her community, and her church who and what she is.  That she had produced a gang banger wannabe serial statutory rapist, that SpermGrandPa was a serially adulterous POS and she was a cuckqueaned idiot.  Eventually she learned to crawl under her slime covered rock at the bottom of her shallow and polluted gene pool until she thought the fire storm had passed. Instead of cautiously crawling out, testing the related climate, and staying under the RADAR she would come out blasting.  So, ass baring brutality was immediately applied.  Had she been reasonable, we would have treated her reasonably.  Sadly, due her choices, that was not to be.  

Eventually SS learned the CO, the details of his SpermClan, the Spermidiot's arrest records, etc, etc, etc... and learned to identify the Hag's lies and crap and to shut it down in real time when he was on SpermLand visitation. Eventually her crap cost them my son who asked me to adopt him when he was 22.

Not long after that he went zero contact with them.

So, my advice is zero tolerance, full frontal destruction, and plaster the SKids, the courts, the community, her work, her church, etc... with her choices and how much of a POS she is.  Don't forget to have it all published in the local paper, tag her on FB with it, and every other social media conduit she uses. 

Lather.... rinse.... repeat.

No need to be nasty about it. Be direct, be professional, and brook no quarter.  These types take kindness, tolerance, and disengagement as signs of weakness.

They choose to behave other than reasonably, they choose pain. Deliver on that choice.

As for Karma, it is real and it always collects.  I just made it my position to aid Karma in collecting immediately and constantly.  The SpermClan chose their behaviors, they lived related Karma. It cost them an incredible man in their family.  He chose me, he chose to have our family name on his uniform.  Sadly, there is more to the Hag's living of the Karma she earned. She was fired from her job, the lawyers in SpermLand would not work with her and with them as our ongoing battles continued, half of her Spermidiot spawned GKs want and have nothing to do with her or the SPermclan, #3 of 4 who she raised along with the SpermIdiot, #2, and #4 is a convicted felon serving a long prison sentence.  Her church ejected her. SpermGrandPa has little to nothing to do with her spending his time in his detatched garage sanctuary.  Undoubtedly still stepping out on her.

If they choose toxic, they choose misery. Deliver on that choice.  Make sure the toxic opposition lives the consequences of their choices while you live your best life.

IMHO of course.

Has nothing going on in her life. Except to cause trouble.  And she is trying to do that.  It's sad that instead of parenting her kids and creating a relationship, she only causing trouble.  Your only hope is that other people see what she is doing. You must disengage from her. Block her. Everything goes through your lawer. 
'and best of luck 

I'm so glad I just stumbled onto this site. I needed to find a support group that understands everything I'm going through. Having older sons and now two younger step kids a SD and SS felt like I got my mom title removed and handed a stepmom title. Like wha? I'm still a mom.. I'm so grateful we have a place to vent. I have to deal with a Toxic bio mom and believe me, limited communication is key, especially if you're in the court process. Before sending anything toss it through chat GBT and it will clean it up, I can definitely empathize... we all hit our wall with toxic people. We just have to be strong enough not to let them change who we are. 

Go zero tolerance and bare her idiot ass fully and completely with the facts of her bullshit. Do it socially, legally, professionally, and financially.

Make shure HCBM lives it, the community knows it, and make sure HCBM cannot side step any of her self created crap.  They should be forced to face it at work, at their church, within their own family, in the community, at the SKid's schools, etc, etc, etc...

As the SKids get older they should receive the facts on HCBM in an age appropriate manner. 

HCBM can either crawl and stay under her rock, or suffer. What she feels about it all should not matter in the least for you.  Be confident and bare her ass.  This includes broadcasting the facts about you.  When HCBM flaps her lying lips, smack her with a cease and decist letter or a defamation suit and make sure everyone in her life knows about it.

Lather.... rinse.... repeat.

KISS.

Keep it stupid simple.