I think that the etiquette rule is that you must invite the new fiance/e. Don't worry about them being offended because they didn't make the cut on their own. It's unfortunate if they do, but that's life. Now that they are getting married, they will both get invited to many things that they wouldn't on their own!
I, too, can think of plenty of situations -- geographically distant cousins came to mind immediately for me, too -- where I wouldn't have invited even a fairly serious boy/girlfriend in the first place. Even if I happened to know how serious the relationship was, and likely I wouldn't, for, say, every one of all my cousins' children, everyone has guest list limits. Furthermore, particularly in the case of relatives, it can put unwarranted pressure on the relationship to invite boy/girlfriends, especially where it would mean expensive travel. So be careful.