Dealing with husband and stepkids?

I have just gotten married (dated 1 year) to a great guy who has 2 kids 6 &8 and I have one, 4. I expect to have some parenting issues, different styles, manners, etc.. to deal with but my issue is my husband. The kids have adjusted well and they don't resent me or him, they really like to spend time with both of us. His kids come over everyother weekend and when they are not here we get along fine. When his kids are here we fight. He gets stressed out and blames me for making his kids act up. He yells and pouts. What I am having alot of trouble with is accpeting his misdirecting the anger at me and not dealing with his kids and the problem. I don't discipline his kids but refer them back to their father and I think he is frustrated and feels like he has to be the bad guy and wants me to step in but I am not going to take on the discipline of his kids nor do I expect him to discipline mine. Any ideas?