How to bring up a past invitation?

I think that its appropriate so long as you do it in a way thats not re-inviting yourself. I think that the perfect way to address the situation is to be blatantly honest. Since you did receive another invite, especially with a family you haven't seen and would like to, use them as the ice breaker. Bare with me, as I am going to make up fake names and a situation to describe my example... You could say something like, "John, I don't mean to be rude, but we Laura briefly mentioned a christmas party a few months ago, and I was wondering if that was still happening, because we were invited elsewhere and a RSVP is required in effort to allow the other family to order their food accurently, etc... It's fine either way, I hope I'm not imposing, I just hadn't heard anything since and I didn't want to be rude and not show up if I was expected and I also didn't want to be rude and not RSVP, or, end up being partyless." (Keep talking without pausing, don't give the other family the opportunity to cut in... this way, you can make sure they know its okat if it isn't still an existing invite, as not to create embarrassment on their part)" I think you'll be fine as long as you act nonchalantly, unoffensive, and as if it's no big deal, but rather that you are attempting to make the best of the situation for everyones sake. GOODLUCK.. hope that helped!