I am a Discovery Health Channel nut and I love the show Mystery Diagnosis. There was an episode where this teenage girl had a terrible BO problem... kids picked on her and everyone told her how much she smelled etc. This went into adult hood and finally her Dr.'s realized that she had some rare metabolic disorder called Trimethylaminuria. Maybe your SS is just lazy and smelly, but I guess this is worth checking out???

Here is the link to her story:
http://www.rarediseases.org/oneinamillion/cheryl_marshall

You said "...I do all the babysitting, running around and stressing..."

So there is your problem.

I wrote this a long time ago and I hope this helps with your problem. It may not reflect your exact situation but you'll get the idea:

The situation with my step-kids finally got to the point where I decided that my efforts to raise them was futile.

First I told my wife in private what I was going to do and why.

Then I stopped investing myself, time and money in their upbringing. I realized that by conscious choice I could cease being responsible for them or their actions. Neither would I take credit for how they turned out, good or bad. I simply stopped interfering in their lives. This is not to say if they would start a fire on the living room floor I would not intervene. But if they did not do the laundry as scheduled I ignored it. Nor did I do any of their chores. If the trash overflowed in the kitchen - well tough. Mom dealt with it when she got home.

They could no longer blame me for the consequences of misbehaving just because I saw the misbehavoir. I never told. If they were caught in a infraction it was not my fault. They began to realize that they were responsible for their actions because of what they did not because I caught them.

I would talk to my wife in private if I had issues, but once she made the decision I backed her up 100%.

You will be absolutely amazed at the look on a kids face when s/he realizes you don't care enough to even tell their bio-parent they did something wrong. And you'll be pleased at their future behavoir and attitude towards you.

I stayed friendly enough and taught one how to drive when she was old enough. I took my spouse and SKs camping on occasion when my kids weren't around. I offered the s-kids advise on living in our world and made the obvious comments when it came to safety and such. But I never made it a judgement. May favorite comment was "Do what you think is in your best interest". It sums it all up. What you sow is what you reap.

Once the kids realized I was no longer the ogre they thought I was their attitude changed. I was not their friend, but neither was I the enemy. They came to realize that I had not done those things to irritate them. They in fact missed the things that they had come to depend on me doing. Permission to go to the mall when their Mom was not home? "Sorry, can't do that".

Their mother came to realize that I wasn't over-reacting to their actions. In fact I wasn't reacting at all. She came to understand that she would have to control the situation and she did.

With that things got much easier around the house.

Billions of kids grew up in the world without help from me and turned out just fine.