My first thought is that you made plans with D based on something that A invited you to. You did not make plans with A and invite D along, right? If A changes her invite then you, as the couple who was originally invited, are free to decline.Advice needed!
If you've been paying attention, haha, you know that I've been dating D. Last Saturday, my friend A met him for the first time (all went well, blah blah, no story there). While we were all hanging out, A talked about how she's going to have a get-together at her house this Friday because her cousin B is arriving for a visit from out of state (B is female, I've met her several times, she and A and I have been on vacation together, etc.). A was planning to invite a few friends from work, one of her neighbors, and D and I. Mixed group of guys and girls.
A was traveling for work this week and got back last night. I emailed her a little while ago to ask what the plan is for Friday and she wrote back saying that last night she decided to just make it a girls night because she doesn't have time to prepare for a full-blown thing. So it would just be me, her and B.
My dilemma is that while I understand that things change and she's trying to catch up from traveling, it's sort of rude to invite someone and then basically un-invite them. It would be better to just cancel/reschedule altogether IMO. On the other hand, I don't want to be that girl who ditches her friends for a guy.
However, if B is only going to be there for the weekend and throwing a big get-together is too much for A to manage, then you have a couple of other options. You can throw something at your place that includes D and everyone else on the original invite list, or you can just talk to D about the change of plans (it's not like this was a formal party with engraved invites that were sent up - it sounds like it was half a step up from an impromptu get together). I can't imagine that he wouldn't be understanding and supportive of your desire to hang out with two girlfriends, one of whom you rarely see.