OP, I think the strange thing to me is, if you were genuinely close - then you would just ask her? Why the need for of all of this guessing and thirdhand information? Even the best friend will present the side of the story that makes her look good, you're not going to know what's actually happened.
Your initial post was about 'step-siblings' and as PP pointed out, being a step-sibling doesn't guarantee a close relationship. Fine.
Now you say you're close to this person, enough to meet up individually etc, I don't see how, in that case, they can not invite you and still have such a close relationship. But that isn't related to you being a step-sibling.
It would be extremely odd to not invite you, but then happily meet up with you , and as I said, not mention anything about the wedding. Unless maybe, since she moved away, you're not that close anymore, and she just thinks that the relationship has run its course. You are still tied to her, as a step-siblings but she just sees you as a friend. Surely the relationship is now irrevocably damaged?
BTW I never actually said whether YABU or not. If you felt close to someone and they made it clear that they don't feel the same, YANBU to feel hurt. Anybody would be. @pizzaHeart As PP have pointed out, people don't have the same expectations with regards to step siblings.
It's more about the reasons for not inviting you, that to me isn't clear cut.
Oh and the other thing is, if you were also genuinely close to your other step-siblings... one of them at least would have further thoughts on this surely. And discussed it with you.
Maybe they all know something that you don't?