Gah! When mom says it's rude to throw more than one shower, and then wants the shower already scheduled to be cancelled so that this "one shower" is hers, it's not about the shower at all. I'd almost be tempted to say "Hey mom, I looked it up and you're right about one shower! But I also read that the person who offers first is the one who throws it. So you snooze, you lose".
Obviously, the solution is to graciously accept both showers, but it's still tempting. She's mean and controlling.