What a shame you are marrying a man who thinks his dc don't need basic social skills!
How charming they must be!
How socially stunted he and the ex must be if they don't realise they are setting up their dc to be socially inept/obnoxious?
The fact he doesn't allow you to mention it to them speaks volumes, doesn't it?
You can't be yourself around them and that alone is a false, awkward position to be forced into.
I would get these issues out with him if I were you op, before the nuptials. You could say you won't be putting a penny towards their meals unless you are happy with their behaviour, see how well that goes down!
I would also revisit the joint account thing and ask him to pay in enough for the 3 and see how well that goes down.
Basically, op get all your niggles and issues out in the open before the nuptials.
I can guarantee you won't want to go through with it once you realise a big reason he'll be marrying you is to offload the domestics, get another person to help put his dc through university, pay for driving lessons, get their foot on the housing ladder, holidays, gadgets, clothes, you name it!
Op you don't seem too keen on forking out for the odd meal for them nor do you seem too keen on thinking it through in the long term.
Oh dear op! You don't even like them much and I can guarantee that dc are pretty intuitive and probably don't like you very much either!
So as you seem set on marrying this man and inheriting his dc which then become yours too why not turn over a new leaf and up your game?
Offer them unconditional love, support, domestic help and finances and I'm sure you will be fine
x