Hi tapper, I thought I would post with a slightly different perspective on your situation. I am a step daughter who lived with my dad and stepmum from age 10 til I left home at 18. I think my stepmum could easily have written your post. She was very open about how much she hated me, how awful I was, how her relationship with my dad would be 'perfect' if I wasn't there and how I was the only thing they ever argued about. I have a younger DB but he never came in for the insane amount of flak which I did. My dad did a very good line in avoiding the situation.
When I turned 16 she completely stopped talking to me and kept it up for a whole year which was pretty impressive given we lived in the same house. I was horribly depressed and angry about it and felt completely powerless. Imagine having no choice but to live with the person who hates you most in the world. When I was 17 she decided to go travelling for a year which was bloody amazing from my POV and I thought that they may break up. When she finally came back, just before my A levels, she made an attempt to apologise and explain her behaviour but tbh it was far too late for that. I had at that point spent 2 years literally counting down the days til I could leave home for good, and when I did leave never came back.
It has taken me years of anger and then therapy to get over the way I was treated at that age. I know teenagers are not easy but I was a straight A student and didn't rebel in any huge way, I always worked hard, did sports, played music, did my chores around the house etc. but it wouldn't have mattered to her what I did: she hated me and that was that.
Fast forward 20 years and we are finally on an even keel with one another, she and my dad are very happy together and she is amazing with my kids. But it was pure hell going through it, for all of us. I urge you to please try and find a way through this, yes your DSC are 'adults' technically but they are still young and finding their way and this is a hugely hugely damaging situation for them to be in. You have to be the grown up here: if you have lost respect for your DH due to the way he parents his kids then you should leave at least temporarily. Perhaps when his DC do leave home for good you will be able to mend your relationship, but this is a hugely toxic situation and you have to take responsibility for it.