Of course it's nutty and breathtakingly self-centered. It's also a pathetic plea for your attention. She was trying to get you to react -- either by coming to this weird event (IF it ever happens) and confirming her craziness as OK, or by resisting and giving her an excuse to play the angry and hurt martyr.
Wisely, you did neither (I think -- it sounds as if your brush-off was pretty mild). Good for you.
But do take care. As someone else noted -- be absolutely sure you have a hospital and written birth plan that ensure only you and your husband can be present in the room when you give birth, or I think mom might just turn up and steamroll you at the moment you most want only your husband there. And you mention that you are going to your home area for the holidays -- oh, my, that sounds like a can of crazy for her to open. Once you are physically there, she will probably pressure you hard to attend this ridiculous "shower," figuring that if she nags you face-to-face, you'll cave. Be ready for that; have somewhere else you have to be no matter when the event is scheduled. It's no lie to say, "Thanks for inviting me again, but Husband and I already have plans to see his family." That's true, yes? You don't have to say that your planned outing is not precisely at the same moment as her shower. Just that "we already have plans." Do not let her get to you. Frankly if I were you I'd be sure to control every single interaction with her over the holidays with great care, so that she cannot cajole or guilt or argue you into anything. I hope you're not staying with her!
It's time now, before the baby comes, to be prepared to hear "I thought I'd come stay for the first few weeks to help you with the baby" etc. From her behavior it sounds as if she might try that, but you should nip it now before it even gets started. Be clear that you and your husband want no visitors for the first four or six weeks or whatever time you determine. (And stick to it!) Then if she wants to come, ensure she gets put up at a hotel nearby -- not staying with you! It sounds like she could make the first months of motherhood tough for you if she swoops in claiming she'll "help," so you and your husband need to send the message BEFORE the baby is born that you want privacy.
I know your post is just about the "grandma shower" but I foresee some ongoing problems with self-centered mom hogging the limelight and intervening once the baby is here, too.