K.M.
Oh, the joys of step-parenting. I have listened to my husband remind me that our house is a different environment and we shouldn't expect his daughter to "just change" every time she comes over. She is almost 14 years old and she only knows the divorced life. I have been a part of her life since she was 4 months old.
The last time we had this "discussion", I reminded him that she has been doing this her whole life and he should stop enabling her. I also pointed out that she adapts to her friends' houses, grandparents' homes, school rules, etc., so ours should be no different. I think he might have realized that I have trouble adapting, too, since I am accustomed to having a 9 and a 6 year old running around. It is a change for me and frustrating. It is a constant struggle.
You two need to parent ALL of the children in the same manner. Not all facets of rules apply (bedtimes may be different due to age differences), but all manner of expectation should be the same. If your children need to clean their room and privileges are withheld if they don't complete the task, then that is what happens for everyone. Admittedly, it is more complicated when they are the step-children and don't live with you full-time, but there are repercussions that you (collectively) can facillitate.
He also cannot make you the bad guy. All that does is make them resent you and does not assist you parenting them. He needs to support you in the combined parenting plan with all of your children, not just the ones you two made together.