I think your idea of buggering off and leaving your DH to it is an excellent idea.
Your stepson was beyond rude to ask you to put a wash on for him. Has he not got a machine at home? If it was broken, he’d be most welcome to use mine as a one off, but as an able-bodied adult, I’d be agog at him expecting me to do it and I’d have told him to help himself.
Loads of people are single parents, I was one for a few years with two children, the youngest severely disable, it can be tough, but he sounds like a lazy arse.
And why are do doing all the running around? Is your DH disabled? Why isn’t DSS asking his dad to put a wash on and why isn’t he at least sharing the cooking?
Yes. Next time a visit is planned, arrange to go off to see a friend/ do a course the day after they arrive. Make a huge fuss of the girls that first day so they feel welcome, them leave them to it to have a lovely catch up with grandpa.
It a shame for the girls that their dad doesn’t seem to be doing a great job of parenting them, the fault lies with him, not them.