This thread kinda makes my heart hurt because we had a situation that I wish was different--
For years, we talked about my husband's sabbatical--he earns 2 weeks each year that he can use every 7 years for a sabbatical--it's $ plus time. So we talked about going (as a family) to several places. But when the time came, and we had lived here 7 years, my son was in the middle of a tough addiction. He was in treatment, and we couldn't leave. Every year after that, we still couldn't go--took a long time for him to get well. When we finally did, dd was 13, eldest ds was finishing a summer of college and was not able to come the whole time, and our middle son was sober for one month. I often wonder if he decided on sobriety a month before in a subconscious attempt to be included.
So, everybody except newly sober son got to go to Israel; dd for the full 3 months, eldest for about 2 weeks, and me and hubby of course for 3 months (hubby also took 2 weeks by himself).
It felt really sad. We just couldn't take our middle son, in fear of him acting out and then being caught and put in an Israeli prison, you know? It is still so sad to me that we couldn't take him. We gave my eldest money for his graduation from college, and said he could spend it however he wanted--he chose to buy a plane ticket and come with us for the two weeks, and we paid for all his costs beyond the 900 dollars. We just couldn't offer the same to our other son.
Was it fair? Probably not. It caused a wound that hasn't healed, and I hope one day will.
And Crimson Wife--this is funny, but we were able to go camping and on trips around the country (like driving up the coast to see family, and a couple of Colorado trips, and Disney) with our oldest two before our girl was born. Not fancy, but good vacations. She got to go overseas. She got to actually live in another country for 3 months. She also got yearly trips to NC to see family. While she isn't resentful, she does wish she got the camping trips and the trips we took with the boys! So it can go either way. :-)
OP, you have a weird situation. It is not the same as my sabbatical story, I just wanted to share that. I guess the similarity is that my son was not "deserving." Your step kids haven't "earned" this trip. I'm not sure that I see vacations as earned, more really a grace, but I do understand your feelings. I guess I feel it would be kinda awkward because they actually live with you; if they were on their own, I wouldn't hesitate at.all. to not invite them.
Perhaps, moving forward, no matter what you decide, it would be a really good time now to help them make a launch plan. IOW, I think you should sit down with your husband and come up with a team-approach plan to get them launched. You could start by setting a date for exit. Then decide on a reasonable rent. Make it enough that they have to work to pay it (and honestly, although I know some people do this, I would not save it and then give it back to them as they leave as a gift). You could ramp it up so that it increases from something rather small (so they can have time to hunt for a job) to what is the going rate in your area (so that they can practice budgeting).
Also, just curious--do they have some executive function or ADD or something else going on? It may be more than laziness and entitlement. But IDK--that is another discussion.
Mostly, I'm sorry this is so hard.