God, it's so hard isn't it. I have 2 ss age 20 and 17, but have known them now for 8 years. They were never rude or argumentative with me, but it's still hard. They live with us full time, and always have. They didn't want to go and live with their mum, and don't see her very often.
I think yours may be partly an age thing, they have both got much better as they have got older, and are really nice kids .
I did read somewhere on some step parenting site, about absolving responsiblity, (I,m not sure that is totally correct!)
Anyway it means standing back and waiting for dh to take initiative, dealing with all their problems, and not dragging you into it at all. This is what we do, there are still problems of course, but on the whole it has worked. I would never ever dream of disciplining dss ever, and never have, even when they were much younger.
Don't know if this helps. I think the hardest thing is when they never visit absent parent, and are with you nearly all the time. Sometimes you want a break from your own kids as well as someone elses!
I do not view myself as any sort of mother to them and never would, ever. They have a mother already, and my role is more of friend or mentor. I suspect that may be where some of yoyr problems are coming from. If they already have a mother (even if they rarely see her) you cannot adopt that role. Try standing back and being more of a friend. I know this is an approach endorsed by stepfamily experts