The first thing you should be doing is talking with her to figure out if this is even a 'boyfriend/girlfriend' situation. In other words, is it no longer OK to date other people? Secondly, (I may have misread but I'm too tired to go back and reread here) why aren't you invited? I trust and love my girl but if she ever told me that she was going on a vacation with mixed company, I'd be very curious as to what was up.
The problem is your timing. Dating a month isn't really long enough to be expected to be invited to everything or start making demands on her. I guess like others have said, wait and see if this 'holiday' ever transpires and if it does a few months down the road and you STILL aren't invited, I'd say the relationship's over.








