I'm not sure I should reply, as you are seeking answers from people with adults SCs and mine are only teenagers! On the other hand, DH and I have been thinking ahead to the not-so-far off time when the teens have left home and started their own lives, and there are definitely worries about how that will feel for everyone.
You say that your stepmum sways between trying to make you welcome, and then seeming to resent you. Obviously, I don't know what her attitude to you is, but simply based on what you have said here, I wonder if maybe you could be misreading some things? If she is welcoming most of the time, and you are seeing them several times a year (which to me does not seem unreasonable given the distance) is it possible that you are seeing as 'unreasonable' or personalising things which she'd see very differently?
[Her decision not to visit you because she 'doesn't like cities' does sound off, though. Possibly there was something else going on there?]
I think the bottom line is that you live at a distance from your family. I do myself, and have almost continually since I became an adult. It is not unusual for people to move away from the family home, and I think it is a healthy thing, but it is hard. Can I ask if you moved away from the famly home, or did they? Once I left home, I had to accept that my parents and other siblings were sometimes doing things together that I could not attend, or that there were times when I could not make it to a gathering because of money, time, or my other obligations.
If your stepmum is deliberately and maliciously scheduling things in such a way that you clearly could not make it, time and again, that is one thing, but I'd also say that she (and your dad, because surely he is just as much to blame for the scheduling of events??!) isn't responsible for making sure you can attend every time. She is inviting you, and if you sometimes can't make it, I think that is probably fair enough. (As I say, if there is somethig else going on and she is truly making it impossible, then you do have a reason to be upset, and I would suggest discussing it with her and with your dad as adults.)
Your stepmum may also just be trying to get on with things with the other kids - not to exclude you, but simply in the reality that you are no longer a kid, and that there are things they will do as a family unit of "parents and juvenile children". You say you live completely separate lives, and if that is the case, that is clearly a problem that needs to be addressed; maybe there are things you can both do to make it more possible to spend time together?
DH and I have a much younger DD, in addition to the DSCs. My DH and I had a conversation recently about future holidays. He was saying how there was probably only one more summer when DSD would be coming on holidays with us, because then she'd be off to university and wouldn't want to come on holidays with us. I told him I thought it was quite likely that DSD would continue to want to holiday with us and her siblings! But I also think that DSD will have a 'life of her own' at university, and begin to plan holidays with her friends, boyfriends, eventually with her own children...in ten years time, when our DD is 12, and DSD is 27, will she still be coming on holidays with us? It might be nice, but I fully expect we'd be making plans for holidays without working around DSD's schedule and convenience.
I do know that we will have some conflicts over this in the first couple of years after DSD has moved out - DSD is likely to expect that if she is going on holiday with her friends, for example, that we will avoid scheduling the family holiday for that time. Whereas I feel (and my DH feels the same, I should hasten to add!) that we need to be able to schedule holidays in a way that suits us and the minor kids still living at home, and that if DSD is living as an adult and making her own plans, we can only go so far to accomodate those. If she is living at a distance from us (which seems very likely), then there is only so much we can do. I hope she'd always feel welcome and a part of the family, but the fact is that an adult child is in many ways separate from the 'at home' family. I'm not trying to dismiss your feelings, but I wonder how much your stepmum is entirely to blame for them, and how much it is just the sad circumstances of living far away (which for me personally is the greatest unhappiness in my life; I don't have child siblings still at home and I am sure that just makes it even harder, so I feel for you).