The OP didn't say in any shape or form that she is making her DSC feeling unwelcome. More over she said that she tried to involve them, but they chose to run off in a strop.
This sounds more like their fathers failure, who leaves step mum alone with DSC and expects her to babysit, and fails to instil basic manners in his children. If my DSD would be behaving in such an ungrateful manner, there would be a reason for a strop in time.
The OPs invite to the DSC to play with her DC was perfectly nice and the fact that DSC went off in a strop isn't the OPs fault. The situation is to blame, and there goes: You can't fix what you didn't break.
While children who experienced divorce certainly have their burden to carry, what parents must do is provide a sense or normality, i.e. no special treatment, as treating them special, or even if they have experienced something abnormal (divorce really is normal these days, isn't it.), would most certainly produce a entitled and spoilt adult who thinks that the world owes them god-knows-what. (Read: Disney Parenting)
Contrary to what some ladies here seem to think, a step parent doesn't owe the DSC anything either, it is his or her choice what she gives them. Marrying someone with children does not change that. They are not the children of the step parent. It is of course better if the step parent and the children like each other, however even that does not entitle anyone to anything. After all they already have two parents who (supposedly) care for them, and if you actually ask most DSC, they want their parents, not step mum, no matter how nice she is, and no "making them feel more welcome" (by doing what? throwing candy at them? letting them do whatever they want??) is not going to fix the way they feel.
Yes, these children are unhappy, as what they want most in the world (mum and dad being back together) is not what is going to happen. Part of life is to learn that you can't always have what you want, and that feeling bad doesn't entitle you to special treatment. The way they feel doesn't give them a free pass to behave like the princess on the pea and disregard all basic manners. Here it is the parents job to ensure that these are learnt.
To the OP: I'm sorry you feel this way, you must feel horrible to have two persons in your home who you're trying to feel welcome but who chose to disrupt your normal life instead of joining in. I would imagine them being upset about the upcoming wedding, and hence looking for attention. You need to chat to your partner, he needs to do something about their behaviour (e.g. explain that this isn't acceptable, but also reassuring them that they are an important part of his life still and that this won't change). Also take into account that children will not do what doesn't work. Meaning: They are getting something out of being like this. Are they behaving like this a lot? Do they get attention from it (this can be positive or negative)? If so, explain this once to them with your partner (or just him) and then ignore the behaviour.