Caorunn are you so confident in your DCs behaviour, manners and conduct that you do not expect their stepmum to be in any way impacted by them while they are living in her home? Are they "speak when spoken to" perfect DCs? Or are they typical kids who grow out of one phase and into another?
My DD is a typical teen, and that includes being very challenging to live with at times. She plays her music too loud, she leaves dirty knickers on the bathroom floor, she drinks the last of the juice - and yes, her dad pulls her up on those things, but her stepmum is tolerant and willing to accomodate her despite, as you say, not having any obligation to do so. They live as a family - her Dad doesn't watch her every move, or check up on her to protect his DW from DDs mistakes. If DDs stepmum goes into the bathroom after DD, it's her that finds the discarded smelly socks, not DDs Dad.
What do you think DDs stepmum should do if she took your approach and wasn't prepared to tolerate/accomodate DDs perfectly normal teenage behaviour? If she felt no obligation to overlook the dirty laundry, wet towels and unmade bed? The only solution would be for her to end her marriage - and that is something that would have a HUGE impact on DD. DD would lose her home, her pets and, of course, her stepmum - she'd lose the life she has now for 50% of the time.
I'd be devastated if DDs dad split with his DW; not only is DD very fond of her stepmum, but she's old enough to feel empathy for her Dad, who would be a wreck if his marriage ended. Surely, all parents would want to avoid that trauma for their child, even if the outcome provides you with the satisfaction of seeing your ex and his OW split?
Why wouldn't I, as DDs mum, be immensely grateful that her stepmum is willing to tolerate DDs normal, but infuriating behaviour, for the sake of her marriage to my ex?
The fact that there is no obligation or expectation on stepmums to tolerate or accept their stepchildren is even more reason to be grateful that they are willing to do so, as the alternative is ultimately damaging to the children.