Well to put a cap on the story, my daughter was delighted to find out she won't be visiting her grandmother on her birthday after all.
My brother was the one who was going to host us (my mom doesn't have the room, and is anti-vax, anti-mask anyway). It turns out some of his in-laws visited on Saturday, stood around talking in the kitchen for three hours (with my anti-vax sister in law, while my once vaxxed brother went upstairs and opened a window), and two of the kids tested positive yesterday. One has a high fever and the shits.