@stepparent55
As other posters have said, this is your husband's doing.
They are HIS children, if he has custody of them it is HIS responsibility to ensure they are cared for, or heaven forbid, care for them himself.
Working all hours to pay inflated unreasonable levels of child maintenance isn't sustainable and he's being mugged off and in turn mugging you off!
As @boringbarbie said, HE can apply for a child maintenance order himself via CMS, to pay her what is legally due. He can apply himself for a child arrangements order to put in place a mutually agreed custody schedule.
There are absolutely no legitimate reason for him not to do these things, I expect he's doing it to avoid conflict with her but it's just not on!
All I keep thinking is those poor kids, a mother who palms them off, a father who works relentlessly and spends next to no time with them and is clearly unable to parent when he does if they trash the house and piss in the beds and rip curtains down as a regular occurrence in his care!
And then a step mother who resents them because she is being run ragged and coerced into taking care of them! Clearly you're a more decent person than either of their actual parents because you actually ARE looking after them, but who is actually loving and nurturing and PARENTING them?!
OP - the saying "what you allow is what will continue" is absolutely spot on here, as is a quote I heard recently "if you think someone is ruining your life, you're right, its you"
Only YOU have the power to improve your situation here, there are any number of ways you might do that... it does mean you might have to make some compromises or difficult decisions though!