What a horrible, rotten, b!tchy, **$#!@!!( thing for her to do to you. I'm so sorry.
I agree that I wouldn't even acknowledge this, I'd never mention it again....maybe she'll wonder when you don't say anything about it. I might even consider lying if she were to ask and let her explain her actions. But then again....she's not worth it.
Congrats on the new grandbaby....sorry it took her petty actions to mar the moment.