I think you need to stop, I mean STOP reacting and THINK long and hard about what this is doing to you. Have you had any counselling at all? A friend, relative, or someone who has been in similar situations?
We had juvenile delinquent teenagers, one of whom ended up incarcerated for two years for battery on the infirm, the court ordered family counselling...TWICE! This kid was beyond the pale!
I really did not like when DH interacted with BM, she's a manipulative (like mother like daughter) bitch and she pushed all the buttons she knew DH had and it looked like they were together still on some occasions because SHE made it look that way. It did not help that DH was so far into denial that he gave them all what they wanted, including BM, and the reasoning was 'for the sake of the kids'. He knows MUCH better now though, with counsellig we've shifted the dynamic some, he can see my perspective and I can see his.
The dirty underwear? Guys do not wash their hands before, some do not even wash after they go to the bathroom iether! I would not make that an issue right now unless you know exactly what the scent, stains or whatever actually are.
It's ironic that the more you think about a bad situation happening the more likely it is to happen! Kind of self fulfilling prophesy thing.
It really sucks, big time, that BM will be in the background until the kids come of age. Even beyond that with graduations or weddings etc.
From experience I can tell you that this girl will stop at NOTHING to get you out of her way, she cares nothing about you, her father, siblings or any family members...all she cares about is HERSELF and her ability to manipulate daddy. Of course her mother is going to encourage her! PAS is the weapon BM's use, and it looks like they are succeeding...please stop letting them get inside your mind...PLEASE!!
Believe me, I thought I was going to go out of my mind at times too, with the bullcrap here but I stopped and considered the reality of the situation and figured out what was really going on. The BM was using SD's loyalties, twisting them and really screwing with the girls mind and it was all to create drama and trouble.
A counsellor gave me the most wonderful piece of advice, she told me that the simple solution is NOT to allow my buttons to be pushed. This woman and this child found buttons I did not even know I had! I looked upon this as a learning curve, a voyage of further introspection if you like. I never did dignify a tantrum from a child, EVER, and this girl knew this so she really had to push the envelope to get a reaction from me...which meant her behaviors were more extreme...also more destructive. I was worn out!
I disengaged from the kids and re engaged with DH. If they asked me for something I'd send them to daddy, they are absolutely correct when they say I am not their mother. They decided, yeah really, that they did not want me to parent them, I decided that they could save face it they wanted to by it being their choice. They pretty quickly learn that they are shooting themselves in the foot (chuckle) and DH learned how badly behaved they really are, how demanding, and time consuming. Previously he left it to me and they bitched at him constantly because they did not like how I did things...have at it buddy!
I think what happened with me was that I got caught up in a 'role' as step mother and forgot about being an individual person. I think I lost some of 'ME' in the process and now I have ME back again life is so much sweeter. It's overwhelming, I am a strong woman but I still got played by a 13 year old girl...OK so she is a sociopath and she had help from an experiences manipulator. Very few people are immune to these sorts of people, given that most of us have a concience and the ability to 'make allowances' or reason that 'they've had a hard life' or any of the other bullcrap justifications that a sociopath would put forward as mitigation for their wrongdoings. My old Grandmother, God rest her soul, used to say of these people 'Well, he/she's PLAUSIBLE. Sure enough!