I'll try to make this brief.
My eldest lives with his girlfriend (he's 24). They live about 4 hours away. We are planning on spending Thanksgiving weekend at my childhood summer home - 2 hrs from us, and 6 hrs from him (with lots of family). While we know it's a long trip, we would really, really like them to come - we see him so rarely. If he doesn't come, he'll go to her family for Thanksgiving (also 4 hours from where they live).
I have spoken to him about this and he said they haven't made their plans yet. That's OK - not everyone is an organizing plan-ahead control freak like me. Here's the issue - he tends to take the easiest route, which is probably going to her house. The King really wants him there (as do I). I have a friendly relationship with his girlfriend - we're Facebook friends and exchange Christmas gifts, etc. - and I know he'll be happier if she comes (rather than splitting up for the holiday).
Would I be out of line to talk to her to make sure she knows about the invitation? I want her to know she's welcome, but don't want him to think I'm 'butting in' or managing him. Am I trying to swing the decision my way? You betcha. But I don't want to cause any problems, because that is rather against the point. I'm OK with splitting the weekend with her family BTW.