I have a 16 year old SS and two small bio sons.
Here's what I would suggest.
Let her spew whatever crap she wants to and don't let it bother you.
My fiance's ex has been spewing all sorts of crap about him for years. He just lets it roll off him like it means nothing. Because really? It IS nothing. Don't let anyone know it bothers you. Don't ask your step son about it (because you must know he probably mentioned to mom that you talked about it...and now she knows it bugs you...) If you let it tear you apart, she wins! And for what? For spewing crap!!! I don't let BMs crap towards my fiance get to me at all. Its just crap. Its not real. Its not true. It doesn't matter. And you know what??? Since she realized she wasn't getting a reaction??? It stopped!!! Now things are calm and lovely.
Have DH tell BM or SS that this is when he should come over and if there are any changes to that, then DH should be notified. If DH is not notified of changes by XXXXX time then the family is free to make other plans. It may take a few times for SS to get that this is real...but if he misses out on something in the meantime and BM starts yelling, hang up the phone. Even better...put the phone down and let her think she is still yelling at DH. HAHA :-)
Bottom line. This is annoying? Yes. But should it be harmful to your relationship? No. Let her spew whatever junk she wants to. You can't stop her anyway. If you stop letting it bother you, it will eventually stop!! You guys are feeding this behavior. Find something that centers you and when you get tense about it, do that thing...and eventually the tense feelings will fade. Its not worth fretting over.
Hope this helps,Jen