My step children are hideously selfish and don't listen to a thing I tell them. They won't even bathe! They're 14(boy) and 11(girl). My husband has a passive parenting style. He hates to hear any fussing or fighting and likes to avoid confrontations. He knows that they treat me bad, but he says he doesn't understand why he lets them do it. He says he just doesn't know how to make them do the right thing. He's tried punishment, rewards, and a combination of both. Nothing seems to work. I know that he loves me very much and it bothers him that he can't figure out how to make the children behave and do as they're told. He also loves his children, but they are tearing our marriage apart. Anyone have any ideas or have you ever been in this same situation??
A little more info:
We have a two year old daughter together. Mu husband just deployed to Iraq and will be gone 15 months. The mother rarely calls, lives out of state and is schizophrenic . She sees them maybe once every other year. My husband is a good man and a good father to my baby. He carries a lot of guilt about his divorce and the impact it had on his kids. I didn't mean to make it sound like he was spinelss. I think his passiveness stems more from guilt than anything else.
We've been married for about 3 1/2 years.
I tried for the first two years to have some sort of relationship with the two of them. I've done things with them/for them, I'm the one who takes them shopping, I'm the one who buys them new clothes, I'm the one who makes their father take them to the library, the park, etc. After two years of doing for them and getting nothing (respect) in return, I stopped doing so much for them. They are just REALLY selfish children who were used to being the bosses and getting what they wanted. I've even suggested family counselling, but now that my husband is deployed, it's too late for that.
Just a little nore in defense of myself: My husband is in the military and he's not home much. The children are left in my care. I have provided them with structure and activities. It's easy for a few to point the finger at me and say I'm the one being selfish, but if you any idea how many "talks" the kidsa and I have had about our relationship, you'd understand why, at this point, I'm pulling my hair out. I agree with some of the negative comments. You're right, I shouldn't let their behavior bother me, but I don't know of any way to just "turn off" my emotions when they treat me so badly. Even their father ADMITS that they treat me badly. Again, I have suggested (on several occasions) that we seek family councelling. I work, their father is gone, and I'm taking care of all three children. Is it so much to ask, for them to just follow a few rules? I've compromised a lot. I stopped nagging them to bathe, brush teeth, do homework or clean up after themselves!
By the way, Skidoo, my screen name was a joke between my sister and myself. One I've had for years. No hidden meaning whatsoever. 
And for any of you who took offense to the term "demonic step children", come on! It was a phrase to show just how frustrated I am with them! I've done more for those two chilldren since I've known them than either their real mother or their father. I've taught them why they shouldn't lie or steal, how to have compassion for others, how to take care of themselves, and a lot of other real life skills they need to become productive adults. The issues they have were going on a looooong time before I was ever in the picture. I tried to give them what they needed but they rejected me, so don't point fingers at me for getting tired of trying to help them when they don't want or appreciate my help. Sometimes, I think the only ones who really understand are the ones who have walked in my shoes. And some called ME judgemental?
Read the out of print book "Love and Power in the Step-Family". You can get it interlibrary loan, and maybe from www.abebooks.com. It outlines the various families(power structures) you are dealing with. Get your husband "PET Parent Effectiveness Training". Use that to help him to become the primary parent. You need to contain discusions to the privacy of the two of you, so that you can elevate your own relationship above that of him and the children, or of the children themselves. Learn only to speak for him in his absence, or to defend yourself from abuse. Use statements like "Your father wants you to…" and let the consequences fall from him after you have been able to talk with him privately. There are other blended family books out there if you have more time to read, but this is the best.