I think the OP mentioned that they took the GF on a ski trip a few months ago, so I don't think it's that unreasonable to want to spend time together as a family without any SO's.
He's 21 though. An 18 year old sure understandable that one trip significant other comes and another trip not.
I will say I understand the position the OP is in. I lived it with my husband's family. I am however coming in from the significant other's position rather than the mother of the son. Anyways my husband's mother had a very very very hard time letting him go (and still hasn't completely but it's sooooo much better now).
I met my husband when he was 18 and I was 19. Every year his family went on vacations which was something that I didn't do with my mom and did a few trips with my dad growing up but never as often as my husband's family or to as nice of places (in general that is).
I saw how difficult it was for my husband's mom to understand that:
1) he was no longer interested in leaving me behind after the first trip and
2) he wasn't up for going if his mom wasn't up for me going (which admittedly I don't know that she ever didn't want me to go aside from the first yearly trip of my relationship with my husband when it was honestly still pretty new--they went to Key West that year, if there were any other trips I was unaware I was not quite invited) and
3) he wasn't up for going if I was unable to go with respect to time off from work and expenses
It's understandable that to the OP this is a trip by their own words has been planned since before she's been around but I can also see how that could be hurtful to the girlfriend if it was phrased that way to her (not to mention the comments regarding the girlfriend not making her son a better person).
Expenses aside (in terms of who is paying or not) this may be more of an understanding of how adult children branch out and make decisions with concern to other people in their lives not just their own blood relations. It may be even harder for the OP if the other older siblings haven't quite done this yet with the yearly trip.