I think YABU.
I suggest that they do not see you as a mother figure in their life, they see you as their father’s wife. If that is true- and from what you have written, you are absolutely aware that your relationship is not on a fond/mother figure footing- why would you expect it? They don’t feel like that, you know it and they know that you know it- so what would be the point of it? It would have been fake and a hollow gesture. It sounds self-defeating to have decided this was something they should do, even though you know they don’t feel that way.
I am not saying they should not be polite to you, or appreciate when you do things for them that you don’t have to (such as paying for a holiday- though they may not know that you paid and their father did not). Though, as you chose to marry a man with children I think you do sign up to do things for his children to some extent too- and to welcome them in to your home (as your husband has with your children- and it sounds like he has done). Some of the things you are talking about are not really a favour you are doing them- you might be doing your DH a favour, but not necessarily doing it for your stepchildren
For example, if you are cooking a meal for you and your DH and your children, you aren’t really specifically going out of your way to do them a favour by cooking are you? They are reasonable to expect to be able to eat a meal when they are staying with their father, in his home. If you are doing anyone a favour, it is your husband, in my view. Depending on their ages, you doing their washing could also be more of a favour to their father than them. If you no longer want to do these things because you don’t feel appreciated, take that up with your husband, who will have take on the tasking of making separate meals for them/ask them to make their own meals, and do their laundry or ask them to do it. If you don’t want to pay for them to come on holiday, then don’t- accept that you and your DH may need separate holidays if they can’t come with you. I’m not sure that will help the situation, but at least you won’t feel put upon and resentful. I’m not trying to be facetious- I think you do have to accept that some of these things you are doing for your husband and your relationship with him/because you want to make life easier for him and because you recognise that in order for him to have a relationship with his children, they have to be welcome in their father’s home, rather than solely for the benefit of your step-children.
I don’t know how old all the respective children are, but I think it is worth bearing in mind is that if your children live with you full time (or most of the time) and they are not resident with their dad, there may well be quite a lot of resentment that your children spend more time and thus indirectly more support/more consistent relationship with their dad than they do. They may be quite unhappy, even if they can’t say it (don’t want to upset dad- e.g. they may worry he will simply distance himself). You and your DH chose this situation, none of the children involved did- you don’t get to decide how they feel about it, or what kind of relationship they want with you. It is incumbent on you, as the adults who decided to try and blend your families, to accept that whilst you might love each other, your children may not feel the same way about your spouse. They must be polite and respect any rules within your (you and your husband’s) home, of course, and that could include being responsible for their own laundry if your children will be at a similar age.