Should I Be Pissed Over This?

The title to this post is not a rhetorical question.
I'd seriously like to know what you think about this:
A few weeks ago, Kevin and I decided we were going to throw a costume party on the Saturday before Halloween. I've been all excited because I'm thinking about going as Foxy Cleopatra, so I'm picturing myself in big Beyonce hair! YES!
But I digress...
Anyway, so we've been telling our friends who live around here and our family, just to give everybody time to get their costumes together. I told my cousin about it last Monday, and she seemed happy about it, and was like, "Oh, I'll have to get a babysitter...BLAH BLAH BLAH." Keep in mind that we all hang out with the same group of people for the most part. So today we go walking, and while we're in mid-stride she turns to me and says, "Did I tell you we're having a Halloween party on the Saturday before Halloween...oh no, is that the same day you're having your party?" I'm taken aback by the whole thing because she obviously knows the answer to that question, and before I could say anything, she goes, "You could just cancel your party and bring everybody over to our house and we could have one BIG party if you want."
I wasn't that upset about it at first, because I'm thinking that it could still be a fun time and it would be nice not to have to go to the trouble of planning the party and decorating, etc. But when I told Kevin, he was a bit upset. See, this was not an isolated incident...it actually happened another time back in May:
It was Memorial Day, and our friends Jeff and Tracy were planning a barbecue. They had spent a lot of money on the meat, and had been cooking briskets all night in preparation for the grand event, so of course they were really excited about it. Anyway, long story short, my cousin decides that everybody would have more fun at her house, and she calls Tracy and is like, "So there must have been some sort of miscommunication, because I was under the impression that we were all meeting at my house." Tracy was not a happy camper, but she graciously packed up the $60 worth of briskets that they had cooked over night, and took them with her to my cousin's house to help feed everybody. The party ended up being stuffy and a bit boring, but again, I digress...
So anyway, I'm not sure if we should go on with the costume party as originally intended, or if we should cancel it so as not to interfere with Stacie's party...