Yeah... .
When the kids were little and a "favor" was asked, even when it benefited everyone involved, the expectation was pretty low of there being accommodation.
For the pwBPD in my life, I think it feels good to say "no". That's empowering and she feels pretty powerless most of the time.
I've found value in not letting her usurp this kind of empowerment by 1.) Not asking for accomodations and 2.) Not reacting when she feels she needs to prove her power. As long as the kids were fine and she wasn't too out in left field --- I usually just chalked it up to her having a moment.
Your stepkids' mom is probably still reeling over all the drama she caused with the holidays.
When my stepkids' mom is reeling, I try to become relatively boring. I don't engage, I don't react, and I don't put out any targets for her to jump on.
Your husband is a rock star for making sure he talks to his kids on a regular basis no matter the obstacles mom wants to put in your way. My husband was crafty when it came to the same situation with his ex. He was scheduled to talk to them every single day and she would do everything imaginable to impede that phone call. So he just walked into the daycare with the court order and asked if he could call the girls every morning at a scheduled time to help reduce the conflict it was causing. They accommodated him completely.