I agree you can't be expected to be the parent, however, you are the adult in the house and the primary care provider of the children. If step sons actions would not be accepted as reasonable at a childcare providers then they are when he is with you.
If it is a reasonable request I expect that there will be compliance, just like the kids expect that money they have asked for to stop after the game for something to eat of whatever.
I demaded respect from all of our kids, his as well as mine, however, that is about as far as I went with his children, they have a mother, and a father, there was no way that I was going to win.
So I spent my time and energy on my own children, and I raised my kids with the life isn't fair, get over it, approach. So even though I made my children do certain things that my step children didn't have to do, they knew their father and I had the belief that it was our responsibilty to raise them to be self supporting productive members of society.
My kids knew what the expectations were as well as potential consequences for non compliance, they also knew that dad & I were the adults and we did not negotiate with terrorist.
Dad should support your request for respect and compliance to reasonable requests, but after that he needs to make his kids responsible adults, you can do it for him.
Good Luck, goodness knows that this is a tough issue.