I totally know what you are going through, but it's with my own 11 yr old son. Of course, he loves me, so sometimes he gets a wild hair and does the NICEST things for me. I'm sure you don't have that LOL.

As far as getting him to do things, try for a few months to have him do things with you. It doesn't matter what it is, you can do the task together, and both take turns doing certain things so that he can watch how to, or you can help him if he doesn't know how to do something right. This is what he's NOT getting from mom. My son is highly intelligent, but he does some of the STUPIDEST things. I have wondered if he does it on purpose...I don't think he does. While you are doing things together, you can engage in small talk. And try to be a little lighthearted, goofy, whatever is easiest to do. He needs to know that he can relax around you, even if he is "working". Once he does this for a while, you will know he knows how to do certain things and HE will know he knows how to do them. Let Mom know ahead of time you are going to have him help you, so when he looks at Mom, she will support you.

What you DON'T want to do is complain that the kids aren't doing X,Y, or Z. It's because Mom did not expect them to or show them how to. Mom is actually the problem here. As a single mom I know it's a lot easier to let them play video games all day and do everything myself. So it's something we have to force ourselves to do...teaching our kids to do them.

My bf was not taught many things growing up, I can tell. His mom died when he was 19 and was sick for a long time before that. I know what it's like living with someone who just doesn't do or know to do certain things. It is hard! LOL So the more SS knows how to do, the less his future wife will have to deal with. Same goes for the SD. FWIW, just because she has 3 kids doesn't mean she even comes close to being a better parent than you. I hope you realize that.

Let it go man. You are twenty years too late to put in anything. Leave it all to Mom. This is what I did:

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.
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 amazed at their future behavoir and attitude towards you.
I stayed friendly enough and taught one how to drive when she was old enough. But 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%.
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 slowly came to realize that I wasn't overreacting 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.

Hi the Panda, these kids sound as if they don't need you to be a dad to them, they just relate to their mum. That's fine so long as they're not actually rude to you. I'd say that following Orange County's advice is sensible. His advice chimes in with years of similar advice by other writers. Just leave the kids to do their own thing and enjoy the adult relationship. If they do things that waste your financial resources or are actually rude to your face, yes challenge them or give them consequences, but if they just do their own thing and ignore you, well that's probably normal for them so don't take it personally. Just chuckle to yourself and be glad that your new partner is not demanding that you have to care about them the way you care about your partner. Now that really would be a bore, having to pretend to like people that you don't like.
Just let it wash over you and enjoy the life you have. These kids will be out soon into their independent life and if the 20 year old moved out quickly, that means 2 more years with the 18yo and only 8 with the 12yo. And how many decades of peace after that?

You are correct in saying the children at home should participate more in the chores. My husband never made his children do much around the house. SD would get up in the morning read the paper(entertainment columb of course), eat and get up go to the TV and leave her dishes for me. Never helped me clean house. My husbands family really likes me they are good people and give me advice. His brother called one day I answered the phone and said" Princes residence Cinderella speaking may I direct your call?" My husband was shocked. I sat him down and talked with him. I told him when you met me you knew I would'nt live nasty. You are going to have to set chores for your daughter or I will. My son cleaned the vehicles, did yard work etc...I told him the princess life is over. She is extremely lazy and your her father. How can you sit there and watch me cook and clean and never ask her to help? He said I was right and apologized. Well needless to say nothing came of this so one day, and saturday was usually chore day since I worked, I handed her rubber gloves, scrubbing bubbles and the toilet brush and told her to clean the bathroom. She said I made plans I said your going nowhere until these things are done. She turned to her dad, I said why are you looking at him? He's not the only one who messes the bathroom. He said, I'll do it. That infuriated me. But none the less i let him do it. I thought how can I get across to her? So I started soaking all the dishes in a big basin in the sink every day she would come in from whatever she was doing I would tell her after you eat the dishes are soaking they need to be rinsed and put in the dishwasher.I knew he hated doing dishes so that seemed to work. But then it set her on a path to get even everychance she could. Sundays I like to cook a big breakfast, a tradition. I'm not a messy cook and wipe up as I go along. One day she got up as usual him also went in and started watching one of their recordings. I said who is going to clean up? He said as soon as we finish SD will do it. I heard the usual huff. I said this needs to be done now because I have to get ready to prepare lunch so if she can't do it now you do it please. Now I have lived through slamming of doors so much it was becoming unbareable. She came out and cleaned up then went upstairs and this is no lie slammed doors about 6 times. I finally told him you need to stop her. Nothing... I went to the bottom of the stairs and told her if she slammed one more door i would take it off the hinges. I heard slam. So I got out the dewalt and went upstairs. My husband came after me and said please don't. I said" Look all I am asking for is a little support from you. You set no rules, no chore list, basically I feel like a slave. I even went as far to put a tip jar on the table. I said you tip the waitresses why not me? He said ok lets figure out a list for the kids. I was like say again. I told him my son is already cleaning the vehicles, doing yard work, and helping with repair projects, what more can u add to his list? He does all this without being asked because I taught him to do so. I said you must treat them as equal as possible. There should be no favoritism. If we don't then this is not going to work. He posted a list for her on the frig. She looked at it making a face. My son was out side cleaning up leaves we have trees in the front yard. I said since these chores are already done you can take a rake and help him. My husband kept quiet. She did it, reluctlanty. I watched the whole time all she did was complain but it started a new pattern around here. You and your wife should sit down and discuss this. If you continue to do everything you'll end up resenting like me. We are now in counciling trying to mend a marriage. One that i am beginning to believe is hopeless