No, AngryBeaver, nobody knows - for sure - what they might be getting into. That's such a sweeping generalisation and I've always felt it is, actually, rather insulting towards those exes and stepchildren (because it's usually rolled out in relation to both) who do make an effort, who do behave reasonably and fairly etc ...... because, otherwise, you might as well say - in relation to any complaint about stepfamily issues - that exes and first children are always justified in behaving badly.
As the OP's explained, her kids behave reasonably and politely towards their stepparent. As do many other stepchildren towards theirs. And of course there are lots of exes who also behave reasonably following a split. It simply isn't good enough to brush aside rudeness, dishonesty, disrespect and so on with "you knew what you were getting into" ..... as if that simple sentence somehow makes all those traits okay. It really pretty much misses the point - like having people be deliberately rude to you is acceptable because you apparently "knew" how things were going to be !
Now obviously, there will be some family setups where stepkids feel justified in being rude or disrespectful .... where they've been treated badly by their stepparent. That's understandable. And I think it's also fair to say that maybe in the early stages of getting to know each other stepchildren might also play up - and the vast majority of adults would appreciate it was due to insecurity and make allowances for a time while things settle down. But years on - when your stepchildren still barely acknowledge you, or when pointed references are made (by kids who are old enough to know better) on a regular basis about "dad's house" .... well, there's no excuse for that. It's plain insolence.
I grant you that only a twit would imagine that taking on someone else's kids would be easy. Most of us fully appreciated it wouldn't be easy .... it stands to reason that there will probably be differences of opinion between how you think kids should be brought up vs either their mother or father's (or both) ideas, you worry about overstepping, you want to be friends - but you don't want to "replace" their mother, you know there'll be practical issues to cope with (accommodating everyone, meals, transportation) ..... I could go on (and on). What you DON'T necessarily imagine however is exactly how unreasonable, unpleasant and stressful situations will actually manifest themselves. Why would most ordinary normal people anticipate scenarios which, at time, are reminiscent of an Eastenders' episode ? If, after a handful of dates with DP, I'd finished things and told him I didn't want to take it any further because I feared that his ex would turn out to be a total bitch and that his kids would become manipulative thoughtless brats in the future, he would have thought me completely mad - and very rude ! Yet, people like me - i.e. stepmothers and "second" wives are regularly told when we complain that we "knew what we were getting into" !! There's a huge difference between being open minded and prepared for some teething difficulties in establishing a new family dynamic and knowing what will actually happen. Whilst adults should of course be prepared to make allowances for children (who of course would never have chosen for their parents to split) just when do you say enough is enough ? Does being a stepchild (or an ex) give you carte blanche to behave badly forever more ? At what age should a child start to accept some responsibility for their own actions - rather than be "let off the hook" because they are a stepchild ? ......
...... there's another thread running here ATM where one poor stepmother has suffered appalling rudeness from one of her stepmother for years, and for no good reason. This "child" is now an adult and still refuses to acknowledge her father's wife's existence. But hey - she knew what she was getting into so she shouldn't object.
I can categorically state I did NOT know what I was getting into. The stuff that's happened for well over a decade was stuff I'd never have anticipated because it was totally outside my sphere of experience. Even now, I still don't know anyone in real life who's gone through similar stuff ..... if the internet didn't exist, I'd feel totally isolated, because it's only through there that I've managed to seek out other women who understand, and who don't immediately write me off as some sort of wicked stepmother stereotype for having the temerity to criticise my stepchildren. My own personal mistake, when I met DP, was to assume that I did have a reasonable idea of what I might be getting into because my own oldest child was a stepchild, following his father's remarriage, and everyone had pretty much got on with the situation amicably and with his best interests at heart - so there hadn't really been much difficulty, and my son seemed content and accepting of the situation. As a mother who'd put aside some of my own feelings in order to make things as easy as possible for her child, I simply hadn't imagined that another woman in the same position wouldn't do the same .... I never imagined that a responsible mother would use her children as weapons for example, nor did I imagine that the same mother would actively encourage bad behaviour from her children in order to "get" at her ex (and me) .... which you make allowances for when they're very young but it's a lot less "acceptable" when you have older teenagers still doing the same thing. So no, I didn't know - and if I had known, I'd have run for the hills.
Anyway - back to the OP ! I know it's very difficult because as you say you don't want to get into a your word against theirs situation .... though frankly, if you're telling your partner that "this" is what's happened, he should believe you - unless you have history of being a liar. I'd lay it on the line with him - say that when he's not around you're subjected to sneers and rudeness - which can't be nice for anyone, them included - and that he needs to try and keep on top of this. E.g. if you have cause to tell them off when he's not there, he needs to accept that you have good reason to do so, and needs to back you up re: consequences (obviously you can both discuss and agree what's suitable punishment for various misdemeanours). He could also try being more observant - many parents are very reluctant to actually see what's going on in front of their eyes because they don't want to "spoil" contact time with discipline. Maybe he could also try talking to his kids about you - not necessarily in a direct "why are you so rude to MrsW" (unless he's caught them in the act, when it would be justified) but more generally - ask them how they think things are going re: the new family setup .... I dunno, just try to get a feel for what they think, and give them an opportunity to air any "grievances" without asking them whether or not they like you. This might then give him the chance to refute any misunderstandings that could explain the rudeness, or otherwise, the chance for him to emphasise that although he doesn't expect them to fawn over you he does expect a minimum level of good manners and respect.
Bottom line though, if, hand on heart, you feel you've been fair to them and have made the effort to include them and welcome them, and yet they're still being rude and, their father isn't doing enough to nip it in the bud, then you must stand up for yourself (and make it clear to DH that if he won't tackle the issue, he can at least support you when you do it instead) and call them on it. Many kids deny whatever it is they're being told off about - doesn't mean they didn't do it. You might find that if you consistently stand up for yourself that they'll snap out of it .... maybe they keep being rude because they know they can often get away with it and they're pushing boundaries, seeing how far they can go.
But whatever the ins and outs of it, it's bloody rotten to feel pushed out in your own home. I hope you can sort something out. 12 year olds should be capable of maintaining a minimum level of good manners .... I'd have felt shocked and disappointed in my oldest if he'd been deliberately rude to his stepmum on a regular basis as that wasn't how I'd brought him up. Unless you're a monster and you've not been honest with us (!!) then you shouldn't have to put up with this.