OP I feel for you. It's a horrible situation to be in and unfortunately there are no easy answers, especially as it sounds like a large part of the problem is your DH's inability and/or unwillingness to deal with it. Also it sounds like it's been going on for years, so it's going to be even harder to change.
To start with, talk to you DH. Tell him how you feel - but be careful to say it's the behaviour that upsets you rather than his children. Otherwise he'll instantly be on the defensive and you'll get nowhere. I'd also tell him that you need his support and understanding. He is enabling the behaviour and, if it hasn't already, that will start to affect your relationship as you loose respect for him. I suggest you decide what you will, and won't tolerate, set boundaries, and stick to them. You'll need to figure out what will and what won't work for you.
For me, every time one or other was rude/disrespectful I'd simply say, "That's really rude and it's not ok". I also said that if they expected me to cook for them, do their washing, be a taxi etc then the rudeness had to stop. I also told DH in advance what I was going to do, and that I needed his backing. For a while he stuck his head in the sand and pretended it wasn't happening too, but once it started impacting on him directly (i.e. he had to be the taxi or do the cooking etc), it changed pretty quickly. If I hadn't it would have been difficult, but not impossible. Needless to say it took a while (I don't think they believed me to start with), but they got the message eventually. Of course when I went through it they were children, not adults, and that made a big difference.
Whilst the situation improved dramatically and was good for a number of years, sadly my eldest SD has once again decided that I am the source of all her problems and a bad person. She once told me she hated me, and it wasn't in the midst of a teenage tantrum either. Such is life. Fortunately she no longer lives with us but when we do get together as a family she pretends I don't exist and I pretend I don't notice. I also pretend it doesn't bother me, but to be honest, it does, but it's just the way it is, so I live with it.
Having said all that, sometimes you are just not going to win, and you need to decide what you want to do if that's the case. Do you want to walk away if their attitude doesn't change or do you love DH enough, despite his lack of support, to disengage and stay?