Hi Jiggerypoke, I think step-parenting is a bit of a minefield. I have been a stepmother for 3 years and I am also a mother. It is very difficult to know what you are allowed to do in terms of discipline etc. It is also very hard when you do not have children of your own. I have 2 small children now, and it's certainly helped now I am a mother.
There is a lot of fear, at least there was in my case of not over-stepping boundaries, of being acutely aware that I was not DSS mother. Is there a father your children see regularly? If so, your partner may not want to be seen to be trying to be the father when he clearly isn't.
I have to remind my DH that DSS has a mum and a dad, and that he cannot defer difficult parenting decisions and disciplining to me. Sure i ask DSS to clear the table etc, but if he doesn't do it, it's up to my DH as the parent to ensure he respects me and does clear the table. If he remains silent on the matter, then DSS will see that he can ignore me and treat me with no respect. I would probably get, "you are not my mum, you can't tell me what to do."
This is my home, I owned it before I met my DH so perhaps it is easier for me to have an input on the house rules. But if I had moved in with DH and he had let the children be a little messy and I didnt like it, i would feel very out of place and uncomfortable changing the status quo, to make sure DSS was more tidy. I would expect my DH to lay out the new ground rules to DSS, and present a united front.
I don't know how long you 2 have been together but the early days are really hard. Your DP isn't going to love your children as you do. It's impossible, especially if there is already a father in place, and your DP has not been active in your children's lives from a very young age.
I don't think you should treat step children as treasured guests, especially if there are other children in the house, but I think that you definitely have to have a united front. If your DP wants your child to be more tidy, perhaps you should honour this wish, it may not be what you would do, but if you were with the child's father you would still need to come to some kind of compromise on parenting matters. I think your DP is looking for you to agree this new rule, because it is important to him, and as it's not really a big thing to get the children to take their dishes to the kitchen, he wants you to enforce the rule. I think once the children see that you agree with DP over this and not backing them up all the time, he may find it easier to ask his stepchildren to do things instead of running to you.
In a nutshell, if your DP would like his stepchildren to do something, and you aren't fussed or in agreement, the children are not going to listen to him, he will be undermined and he will feel resentment at the situation. Small things then get blown out of proportion and your relationship suffers and everyone is unhappy.