I do think you should politely decline instead of ignore. Why put up a wall or burn a bridge when you have absolutely no reason to? A decline of the invite states neither your displeasure nor approval of the invite. It just is. It's an actually neutral response vs. ignoring it.
And I really wouldn't block her number. And I'm not saying that so you can be besties or trade after school snack recipes with this woman. I'm a saver. I keep a ton of information out of an abundance of caution. Retaining the ability to contact OM's sister could some day help you track down your kids or your ex. But that's just me.
